Issue95 inline extensions

This PR is now based on the new Groovy DSL.
It is possible to define extensions inline but also to refer to separate files containing extensions in this DSL.
Tried this but that lets all tests fail:
org.gradle.api.internal.MissingMethodException: Could not find method extensions() for arguments [./build/resources/test/src/asciidocextensions/blockMacro.groovy] on task ':asciidoctor'.
 for File arguments and 
org.gradle.api.internal.MissingMethodException: Could not find method extensions() for arguments [org.asciidoctor.gradle.AsciidoctorTaskInlineExtensionsSpec$_spock_feature_0_6_closure8_closure27@5c5c2fe6] on task ':asciidoctor'.
for closure arguments.