Implement line highlighting for Pygments

[mojavelinux]

Asciidoctor supports line highlighting in source blocks. Now that we figured out how to add background colors in source listings, we should support this when highlighting with Pygments.

Example:

[source%linenums,groovy,highlight="7-9"]
----
package com.example
 
import static ratpack.groovy.Groovy.ratpack
 
ratpack {
    handlers {
        get {
            render "Hello, World!"
        }
    }
}
----