Add Repository.load_filter_list, FilterList

[jorio]

This PR adds support for the main git_filter_list features in libgit2.

pygit2	interface for libgit2 function
Repository.load_filter_list	git_filter_list_load
FilterList.apply_to_buffer	git_filter_list_apply_to_buffer
FilterList.apply_to_file	git_filter_list_apply_to_file
FilterList.apply_to_blob	git_filter_list_apply_to_blob
FilterList.__contains__	git_filter_list_contains
FilterList.__len__	git_filter_list_length

Important note: filter_unregister already exists in pygit2. Unregistering a filter while it's still in use in a FilterList would cause undefined behavior in libgit2. To prevent this, filter_unregister checks that the filter isn't in use in any live FilterLists before proceeding - otherwise, it raises RuntimeError. (To do so, I moved the implementation for filter_unregister from C to Python.)