Allow git-lint to check branches

[dougalg]

This modifies the behaviour (for Git only) to accept --branch specifier which will allow users to get linting diffs between branches.Useful for testing after commits and PRs have been made. This also now gives the ability (for git) to test arbitrary commit distances apart (although that UI is not implemented here)

I've had some trouble running the tests, but I think I'm getting closer to having them fixed up soon. Also, I don't consider this complete, but would like to get feedback on what you think of my approach here before I go any further.

Thanks for the great tool and hopefully you find this concept useful!

[dougalg]

Looks like there's still a couple of failures in the end to end tests. Not sure how to resolve those. @sk- if you can provide any insight?

[atourkow]

@sk- Would like to see this added. Would be super useful to check against PRs in Travis builds.