Stop assuming that repos are owned by the current user

git-hub used to assume that the repo was cloned from the current user's
github, which is not necessarily true. Now it searches for a remote in
the following order: owned by current user, named origin, named
upstream, first defined repository.
Refactor mirror to remove code duplication
Add an option to configure goblet when mirroring
Add 'mirror user/*' as shorthand to mirror all repos of a user
New version