Implement multiple repository support

Would be killer to have multiple repository support, to avoid having to `gush configure` when switching between repos and/or github to github enterprise.

Possibly having `gush configure` be more of a global setting and adding in a `gush init` method that would be run in the top level of the repository's folder.  This would work similar to git's init method and store a `.gush` folder with configs in that folder.

When executing commands, we can check if we're in a gush initialized repo - again similar to git.