Generalize the `GitHub` type

[jsantos17]

We currently expose a very concrete transformer stack. We should try to factor out more stuff out as type constraints instead.

Relevant code: https://github.com/stackbuilders/octohat/blob/master/src/Network/Octohat/Types.hs#L238

We might even be able to get rid of the CPP macros if we generalize enough.