Provide Ability to Override GitHub Url

[CodyRay]

(This is likely a post-expo issue)

A common workflow is to fork the repo to the developers personal GitHub before making changes.

Currently, there is no way for this workflow to be supported by SemDiff. SemDiff would find the personal GitHub repo URL. However, it would have no pull request associated with it.

I propose to add an option to the config file to override the remote found in the git config.

[MRose90]

This shouldn't be too hard to implement as the config file loads after the git repo is found, therefore there is no fighting for fighting for the URL setter.