Utilize go-git-providers for Pull Request creation & SSH URL parsing

[luxas]

At the moment, go-git-providers can't create PRs (yet!). Hence, I just wrote a sample implementation using the "raw" Github provider as an example of a PullRequestProvider. Eventually, this should just be using plain go-git-providers, regardless of backing provider.

Also, when go-git-providers supports parsing SSH URLs, that should be used, at least in the sample implementations.

cc @dinosk @michaelbeaumont @chanwit