Support extension of preprocessor

[homacs]

Hello there!

I thought about extending jcpp to support preprocessing of GLSL (OpenGL Shading Language). This would require to add new preprocessor directives such as "#version" and "#extension", which can be basically ignored in my case. I'm currently modifying the classes as I need them but you might be interested in adding the ability to just extend your preprocessor in a more reasonable fashion for such use cases. Such as adding hooks or just subclassing Preprocessor or something similar.

Regards Holger

[shevek]

I'll gladly take patches; I don't have a great deal of time to work on features on this project right now.