Allow per-filetype configuration

Hey, thanks for porting this over to vim. I have been using expand-region.el with evil and have been waiting for someone to do a vim port. I particularly like how you reused text objects!
Sometimes it would be nice to have per-filetype configuration (f.ex. indentation based text objects in python, `ap` instead of `ip` in clojure/lisp).
I see two approaches:
• Allow the configuraion to be shadowed by a buffer-local variable (that may be set in an `autocmd FileType …`)
• Implement some white-/blacklisting/dictionary magic
I think I prefer the first approach. If you tell me your opinion I will work on a patch.