fixes #206 - adds the option to set attributes through command line a…

…nd updates README

README.adoc

@@ -205,6 +205,36 @@ In the `<attributes>` part of the Asciidoctor Maven Plugin configuration:
 
 You can find more information and many examples ready to copy-paste in the {uri-examples}[Asciidoctor Maven examples] project.
 
+==== Command line configuration
+
+Configuration options can be set (but not replaced) using system properties directly in the command line as follows:
+
+ mvn generate-resources -Dasciidoctor.sourceDirectory=src/docs -Dasciidoctor.outputDirectory=target/docs
+
+All options follow the naming convention _`asciidoctor.` + option_name_.
+
+In order to provide a higher degree of flexibility `attributes` configuration follows a different behavior.
+Attributes defined through the command line are added to the ones already found in the XML configuration.
+The result of it is that attributes and other configuration options can be updated if they are added to the command line as attributes.
+For example, the following configuration could be modified with the command options as seen below.
+
+[source,xml]
+----
+<configuration>
+    <backend>html5</backend>
+    <sourceHighlighter>coderay</sourceHighlighter>
+    <attributes>
+        <toc>left</toc>
+    </attributes>
+</configuration>
+----
+
+ mvn generate-resources -Dasciidoctor.attributes=toc=right
+
+ mvn generate-resources -Dasciidoctor.attributes="toc=right source-highlighter=highlightjs imagesdir=my_images"
+
+Note that in the second case we need to use quotes due to the spaces, and that `source-highlighter` is the asciidoctor attribute name used to update the configuration.
+
 === Multiple outputs for the same file
 
 Maven has the ability to execute a Mojo multiple times.

src/main/java/org/asciidoctor/maven/AsciidoctorMojo.java

@@ -82,9 +82,12 @@ public class AsciidoctorMojo extends AbstractMojo {
     @Parameter(property = AsciidoctorMaven.PREFIX + "requires")
     protected List<String> requires = new ArrayList<String>();
 
-    @Parameter(property = AsciidoctorMaven.PREFIX + Options.ATTRIBUTES, required = false)
+    @Parameter(required = false)
     protected Map<String, Object> attributes = new HashMap<String, Object>();
 
+    @Parameter(property = AsciidoctorMaven.PREFIX + Options.ATTRIBUTES, required = false)
+    protected String attributesChain = "";
+
     @Parameter(property = AsciidoctorMaven.PREFIX + Options.BACKEND, defaultValue = "docbook", required = true)
     protected String backend = "";
 
@@ -384,6 +387,12 @@ else if (val instanceof Boolean) {
                 attributesBuilder.attribute(attributeEntry.getKey(), val);
             }
         }
+
+        if (!attributesChain.isEmpty()) {
+            getLog().info("Attributes: " + attributesChain);
+            attributesBuilder.arguments(attributesChain);
+        }
+
     }
 
     public File getSourceDirectory() {

src/test/groovy/org/asciidoctor/maven/test/AsciidoctorMojoTest.groovy

@@ -858,4 +858,32 @@ class AsciidoctorMojoTest extends Specification {
             !(new File(outputDir, 'github-include.html').text.contains('modelVersion'))
     }
 
+    def "command line attributes replace configurations"() {
+        setup:
+            File srcDir = new File('target/test-classes/src/asciidoctor')
+            File outputDir = new File('target/asciidoctor-output/command-line-options')
+
+            if (!outputDir.exists())
+                outputDir.mkdirs()
+        when: 'set toc and sourceHighlighter as XML configuration and command line attributes'
+            AsciidoctorMojo mojo = new AsciidoctorMojo()
+            mojo.backend = 'html'
+            mojo.sourceDirectory = srcDir
+            mojo.sourceDocumentName = 'sample.asciidoc'
+            mojo.outputDirectory = outputDir
+            mojo.sourceHighlighter = 'coderay'
+            mojo.attributes['toc'] = 'left'
+            // replace some options
+            mojo.attributesChain = 'toc=right source-highlighter=highlightjs'
+            mojo.execute()
+        then: 'command line options are applied instead of xml configuration'
+            outputDir.list().toList().isEmpty() == false
+            outputDir.list().toList().contains('sample.html')
+            File sampleOutput = new File('sample.html', outputDir)
+            sampleOutput.length() > 0
+            String text = sampleOutput.getText()
+            text.contains('<body class="article toc2 toc-right">')
+            text.contains('<pre class="highlightjs highlight">')
+    }
+
 }