Feature: Allow custom GH instance urls to be passed in to auth

[lukigarazus]

I have a custom instance of GH Enterprise, so the default behaviour of using GitHub.com for auth does not work for me, as the code is not recognised.

[zbirenbaum]

I've been working on this today but it's a little tricky because I don't have a custom instance to work against so might take a little time. Once I have something I think works could you test it out and see if any errors come up?

[lukigarazus]

Of course

[egemenkopuz]

I can also help you test it out, just notify us here when you create the branch.