Config: move to cffi

This halves the amount of code we have to take into account for dealing
with the config.

There is a slight change in the API. Config.get_multivar() returns an
iterator instead of a list, which lets us reuse code from the general
iterator and is closer to libgit2's API.
Config: expose config rules parsing

Expose Config.parse_bool() and Config.parse_int() to parse text
according to git-config rules.