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
- [ ] Tests
I'm not sure to follow here. Unless I'm wrong (which is always a very valid assumption :wink:), given `/dir/sub/file.txt`, only `file.txt` would be returned here. Is that expected?