Add base script option to groovyc and its ant task

Please, provide a way to tell groovyc and its ant task which base script should compiled scripts extend from as it is already possible using GroovyShell.

In Gaelyk project we are precompiling a scripts to gain better runtime preformance. Currently, we can only precompile pure groovlets and groovy templates as they don't need to extend any special base script but we would like to be able to also able to precompile other script which relies on having particular base script (routes, plugins).