Configure asciidoctor-gradle-plugin

Based on instructions at:

 - https://github.com/asciidoctor/asciidoctor-gradle-plugin#installation

With the following as "hello world" Asciidoc content:

 - https://gist.github.com/mojavelinux/9a8d3f1d32a8f0656a42

Usage:

 - Run `./gradlew asciidoc`
 - Open `build/asciidoc/hello.html`

See #91

build.gradle

@@ -1,5 +1,16 @@
+buildscript {
+    repositories {
+        jcenter()
+    }
+
+    dependencies {
+        classpath 'org.asciidoctor:asciidoctor-gradle-plugin:1.5.0'
+    }
+}
+
 apply plugin: 'java'
 apply plugin: 'application'
+apply plugin: 'org.asciidoctor.gradle.asciidoctor'
 apply from: 'gradle/javafx.gradle'
 
 version = '0.1.0-SNAPSHOT'

src/asciidoc/hello.adoc

@@ -0,0 +1,14 @@
+= Hello, AsciiDoc!
+Doc Writer <doc@example.com>
+:showtitle:
+
+An introduction to http://asciidoc.org[AsciiDoc].
+
+== First Section
+
+* item 1
+* item 2
+
+[source,ruby]
+puts "Hello, World!"
+