Add the ability to run checks locally

[freakboy3742]

Beefore currently has some implicit assumptions that it's running on a PR that is active, and on GitHub. While this is an important use case, it's also important that we can run against a local git checkout.

[rhyspowell]

@freakboy3742 had you given any thoughts on how you wanted this to work?

I can see that being able to pip install, call with some arguments is possibly the most sensible way but having never done anything like that might be a bit of a stretch for me right away. Was thinking a short step of building it in to be run from a pulled version of the repo and then looking at packaging for pypi?

[freakboy3742]

The issue isn't with invocation - that already works. If you pip install beefore, you can run beefore to start the beefore executable. However, it fails because of the command line arguments that are required.

Those command line arguments request Github-specific details (username, pull request ID, etc), which make sense when you're checking a PR, but don't for local execution. We need to be able to run checks against the "diff against master" for a development branch, rather than using the Github API.

My initial thought is that running beefore in "CI-mode" (which is what is currently implemented) should be turned on with a flag (e.g., beefore --ci -u username ...); "naked" invocation would be "check the current branch against master"; but there would also be ways to invoke locally in the case where master isn't the comparison branch, or master is the branch you're working on.

[rhyspowell]

Ahh I should have checked to see if it was packaged already.

I will have a look, and see if I can put something together and put it out for discussion