Added some logic so it can handle multiple Github configurations

With this added logic you can use multiple Github organisations and/or
Github Enterprise organisations. This code doesn’t change the default
behaviour, but it does change the Github config section from being a
string with a single entry (access_token), to an array with one (just
the one ‘access_token’ entry) or several entries (depending on your
config/setup).

This change needs to be merged in correlation with https://github.com/berkshelf/berkshelf-api/pull/88