Register custom filters

I want to expose an API for registering custom clean and smudge filters via libgit2.

My knowledge of C is rusty, and this is the first time I have looked at libgit2sharp in any great detail. So, I'm going to open this PR early, in the hopes that if anyone has time to spot anything terrible I do along the way they will be able to point me in the right direction :pray: 

###  Todo

- [x] Register and deregsiter a filter
- [x] Define apply callback
- [x] Define check callback
- [x] Define shutdown/init callbacks
- [x] Tests
@ammeep WHAT? *"Sorry"*? I forbid you from being sorry about **code**. Ever. Again. :stuck_out_tongue_winking_eye: 

That's a wonderful pull request. Thanks thousands for it!!!! :heart_eyes_cat: