Make git branches for inch.report configurable

[joshsmith]

Would love to use something like CIRCLE_BRANCH to identify the branch you're wanting to report for. I don't really want reporting on every branch that's pushed to GitHub, especially prior to them becoming a pull request. I just want, say, develop (my default branch) to be the single source of truth for my documentation status.

If I could configure the branches in my settings and additionally have the default branch on GitHub be the only branch that reports (unless I say otherwise), that would really cut down on a lot of the extraneous reporting that needs to happen on Inch itself, cut down slightly on my build times, would be good for the environment, etc etc.

What do you think?