Add processAST option.

I added an option to modify the abstract syntax tree between parsing and compiling.

I liked Alex Sexton's approach to localization in his require-handlebars-plugin project: https://github.com/SlexAxton/require-handlebars-plugin#i18n but wanted the option to easily do this in my build step.

I'm sure someone could find other reasons to mess with the AST as well :)