Allow custom file extension for files generated by @Interface/@Enum

[raner]

By default, all files generated by @Interface or @Enum annotations have a .java extension. It should be possible to change this default extension to something else, probably best with a per-package annotation (e.g., "@Template"?) that might also be used to customize other per-package parameters of code generation.

For downstream purposes, this particular feature might be sufficient replacement for self-type support (issue #33) or custom code generation templates.