Support for github enterprise

[hco]

Currently only github.com is supported, but I would really like to see support for Github Enterprise, which would probably mean making the url to github configurable.
Please be aware that with Github Enterprise, the API has a different URL as well - so probably you would want to make both URLs configurable.

This does probably depend on adding support for authentication.

[hco]

Oh, and by the way: I'm happy to help :)

[espy]

Thanks a lot for that pointer and especially the offer to help! We've definitely got things like this planned for the next version, which will probably be completely new anyway, because of auth and various other things. We have no timing for that yet, though :/

[hco]

I guess this "completly new" point already from #10 - this is why I didn't start to implement it yet.

Feel free to let me know here as soon as it's "safe" to implement this feature :)