Introduce libgit2 options

Allow setting and getting the mwindow size and search paths.

---

This is a first approximation, mapping libgit2's API one-to-one, though I can imagine we might like to have some global where we can set and get in a less annoying way, maybe a global `pygit2.settings` class with setters and getters than we can use, like

```python
mmap_size = pygit2.settings.mmap_size

pygit2.settings.search_path[pygit2.GIT_CONFIG_LEVEL_GLOBAL] = '/var/lib/nobody'
```