Refactor Github specific code into own backend

[isra17]

A first step into adding different SCM backend (I'm aiming to add Gitlab support).

What do you think? I can stack a few PR until I get Gitlab working if you want to hold on merging the refactor.

But if I get a Gitlab backend implementation, would you be willing to merge this?

Ref: #17

[danobi]

Hi! I think the goal is reasonable -- gitlab support would be nice to have. I'd be interested in merging support for it.

However, my main concern would be maintainability. So far I've been pretty light on tests (other than the parser, which basically required tests to implement) cuz I was doing this for fun. But if gitlab support is added, we probably want a way to test the api wrangling bits. Cuz it'll be hard for me to manually test gitlab + github for every small change. I haven't put too much thought into how we'd do it yet but I'm sure there's a reasonable way to get meaningful coverage.

On the tactical side, I'd like to see end to end gitlab integration working before merging a refactor. I'm not familiar with gitlab APIs but I have to imagine it won't be identical to GH. So this would be to prevent unnecessary churn.

[isra17]

Turns out to be trickier, I had hoped for a quick win, but this involve some change in the design such as to track review origins and create a Prr struct from a review instead of the config.

This is doable, but I only had time for a quick win, I will close this and let someone else take over if there's enough interest :/