Feature Request: Allow for custom OAuth authorisation with GReader based API

[SaphireLattice]

Hi! I have been looking into RSS server things, and wanted to set up something that uses OAuth. And while I see the code supporting that in NetNewsWire, I don't think that is ever exposed to the user?

It would be really nice to be able to point the app at a generic self-hosted GReader API with OAuth being used.

Not quite sure how that looks on the API end, but I am guessing there's some support for it? Hopefully.