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
Hmmm. Actually, I don't know. I'm not sure about the memory lifecycle. This looks like being dealt with by unregister. 
Would that still work if we looked up a filter that we didn't previously create?