Issue95 inline extensions

This PR allows to define asciidoctor extensions inline in gradle build scripts as closures.
The test case AsciidoctorExtensionsDelegate.groovy shows how to do that.

The general strategy is to define classes at runtime using a GroovyClassLoader that implement the respective interfaces and delegate to the closure defined in the gradle file.
There is one class per extension closure.

If this approach is welcome I would certainly extend the documentation in the `README.adoc`;-)