Add sections plugin

- Use section and h1 tags to assign steps to sections
- Optional divs showing the section progress
- active-section and hidden-section are classes to mark steps as active and
  hidden section element.

src/plugins/section/README.md

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+Section plugin
+==============
+
+Sections for impress.js presentations
+
+Usage
+-----
+
+Add `<section>` and `<h1>` tags to group your steps as you can see here:
+
+```html
+<div id="title" class="step">
+    <h1>Title of Presentation</h1>
+
+    <p>Our agenda for today:</p>
+    <ul id="impress-section-agenda"></ul>
+</div>
+
+<section>
+  <h1>Section Title</h1>
+  <div id="first-slide" class="step">
+
+  </div>
+
+  <!-- Nested sections are supported as well -->
+  <section>
+    <h1>Sub Section Title</h1>
+    <div id="second-slide" class="step">
+
+    </div>
+  </section>
+</section>
+```
+
+The section name and the current index of the section will be displayed in your presentation.
+Therefore, add a div for displaying the current section and/or progress as you can see it here:
+
+
+```html
+<div id="impress-section-overview">
+    <div class="impress-section-numbers"></div>
+    <div class="impress-current-section"></div>
+</div>
+```
+
+```css
+#impress-section-overview {
+    display: flex;
+    align-items: flex-end;
+    justify-content: flex-end;
+}
+
+#impress-section-overview .impress-section-numbers {
+    display: inline-block;
+    margin-top: .25em;
+    padding: .1em .25em;
+    color: white;
+    background: #aaa;
+}
+
+#impress-section-overview .impress-current-section {
+    padding-left: 5px;
+}
+
+#impress-section-overview .impress-current-section .section-spacer {
+    padding-left: 0.3rem;
+    padding-right: 0.3rem;
+}
+```
+
+Feel free to change the style of your section overview as you like by editing the CSS file.
+
+Additionally, the plugin will generate an agenda outline for you if you want it to. Therefore, add a `ul` tag with the 
+id `impress-section-agenda` to any of your divs of the presentation (as shown in the aforementioned HTML snippet).
+
+Furthermore, this plugin adds the class `active-section` to all steps of the action section and `hidden-section` to all
+steps that do not belong to this section. You can use this classes, e.g. to hide the steps of another section:
+
+```css
+.impress-enabled .step .hidden-section {
+    opacity: 0;
+}
+```
+
+Author
+------
+
+Copyright 2020: Marc Schreiber (@schrieveslaach)

src/plugins/section/section.js

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+/**
+ * Sections Plugin
+ *
+ * Copyright 2019 Marc Schreiber (@schrieveslaach)
+ * Released under the MIT license.
+ */
+/* global document */
+
+( function( document ) {
+    "use strict";
+    let root, api, gc;
+
+    let indexedSteps = null;
+
+    const agenda = document.querySelector( "#impress-section-agenda" );
+    const currentSection = document.querySelector(
+        "#impress-section-overview .impress-current-section" );
+    const sectionNumbers = document.querySelector(
+        "#impress-section-overview .impress-section-numbers" );
+
+    function findSection( element ) {
+        if ( element.parentNode == null ) {
+            return null;
+        }
+        if ( element.parentNode.tagName === "SECTION" ) {
+            const sectionElement = element.parentNode;
+
+            let title = "";
+            const headingElements = sectionElement.getElementsByTagName( "H1" );
+            if ( headingElements.length > 0 ) {
+                title = headingElements[ 0 ].textContent;
+            }
+
+            return {
+                element: element,
+                sectionElement: sectionElement,
+                sectionTitle: title
+            };
+        }
+        return findSection( element.parentNode );
+    }
+
+    function indexSteps() {
+        return Array.prototype.slice.call( root.querySelectorAll( ".step" ) )
+            .map( function( step ) {
+                return findSection( step );
+            } )
+            .filter( function( section ) {
+                return section != null;
+            } )
+            .map( function( step, index ) {
+                step.index = index + 1;
+                return step;
+            } );
+    }
+
+    document.addEventListener( "impress:init", function( event ) {
+        root = event.target;
+        api = event.detail.api;
+        gc = api.lib.gc;
+
+        if ( agenda !== null ) {
+            indexedSteps = indexSteps();
+
+            function depth( heading, n = 0 ) {
+                const parent = heading.parentElement;
+                if ( parent !== null ) {
+                    if ( parent.tagName === "SECTION" ) {
+                        return depth( parent, n + 1 );
+                    }
+                    return depth( parent, n );
+                }
+                return n;
+            }
+
+            const headings = [].slice.call( document.querySelectorAll( "section h1" ) ).map( function ( heading ) {
+                return { text: heading.innerText, depth: depth( heading ) };
+            } );
+
+            headings.reduce( function ( dom, heading, index  ) {
+                if ( index === 0 ) {
+                    const li = document.createElement( "li" );
+                    li.innerText = heading.text;
+
+                    dom.appendChild( li );
+
+                    return {
+                        ul: dom,
+                        depth: heading.depth
+                    };
+                }
+
+                if ( dom.depth === heading.depth ) {
+                    const li = document.createElement( "li" );
+                    li.innerText = heading.text;
+
+                    dom.ul.appendChild( li );
+                } else if ( dom.depth < heading.depth ) {
+                    const ul = document.createElement( "ul" );
+                    const li = document.createElement( "li" );
+                    li.innerText = heading.text;
+                    ul.appendChild( li );
+
+                    const parentLi = dom.ul.lastChild;
+                    parentLi.appendChild( ul );
+
+                    return {
+                        ul,
+                        depth: heading.depth
+                    };
+                } else {
+                    const ul = dom.ul.parentElement.parentElement;
+
+                    const li = document.createElement( "li" );
+                    li.innerText = heading.text;
+                    ul.appendChild( li );
+
+                    return {
+                        ul,
+                        depth: heading.depth
+                    };
+                }
+
+                return dom;
+            }, agenda );
+        }
+    } );
+
+    document.addEventListener( "impress:steprefresh", function( event ) {
+        updateSectionOverview( event.target );
+    } );
+
+    function updateSectionOverview( targetElement ) {
+        if ( indexedSteps === null ) {
+            return;
+        }
+
+        const indexedStep = indexedSteps.find( function( step ) {
+            return step.element.isSameNode( targetElement );
+        } );
+
+        if ( sectionNumbers !== null ) {
+            if ( indexedStep === undefined ) {
+                sectionNumbers.innerText = "";
+            } else {
+                sectionNumbers.innerText = indexedStep.index + "/" + indexedSteps.length;
+            }
+        }
+
+        if ( currentSection !== null ) {
+            currentSection.innerHTML = "";
+
+            if ( indexedStep !== undefined ) {
+                function findSectionTitles( sectionElement, titles = [] ) {
+                    if ( sectionElement.tagName === "SECTION" ) {
+                        const headingElements = sectionElement.getElementsByTagName( "H1" );
+                        if ( headingElements.length > 0 ) {
+                            titles = titles.concat( headingElements[ 0 ].textContent );
+                        }
+                    }
+
+                    if ( sectionElement.parentElement !== null ) {
+                        return findSectionTitles( sectionElement.parentElement, titles );
+                    }
+
+                    return titles;
+                }
+
+                const titles = findSectionTitles( indexedStep.sectionElement ).reverse();
+                titles.forEach( function( title, index ) {
+                    if ( index > 0 ) {
+                        const span = document.createElement( "span" );
+                        span.classList.add( "section-spacer" );
+                        span.innerText = "❯";
+                        currentSection.appendChild( span );
+                    }
+
+                    const span = document.createElement( "span" );
+                    span.innerText = title;
+                    currentSection.appendChild( span );
+                } );
+            }
+        }
+
+        const currentSectionElement = indexedStep !== undefined ? indexedStep.sectionElement : null;
+        Array.prototype.slice.call( root.querySelectorAll( ".step" ) ).forEach( function( step ) {
+            const sectionOfStep = findSection( step );
+
+            let activeSection = false;
+            if ( currentSectionElement === null && sectionOfStep === null ) {
+                activeSection = true;
+            }
+            if ( sectionOfStep !== null && sectionOfStep.sectionElement.isSameNode( currentSectionElement ) ) {
+                activeSection = true;
+            }
+
+            if ( activeSection ) {
+                step.classList.add( "active-section" );
+                step.classList.remove( "hidden-section" );
+            } else {
+                step.classList.remove( "active-section" );
+                step.classList.add( "hidden-section" );
+            }
+        } );
+    }
+
+} )( document );