Per-project configurability

[abesto]

As noted on #22, we're getting to a point where we want to fine-tune the behavior of the checker tool. While every option should still be exposed through the CLI, we should start allowing configuration via a config file hosted in the source repo.

This is different from #20 in that this would need to be a structured configuration supporting various flags (think ~/.gitconfig vs .gitignore).

We still have a bootstrapping problem around the git repo: how do you read the config file if you don't know where the repo lives? Maybe the prudent thing to do would be to allow passing in the full path to the git repo to use. There's also the idea of storing per-project bootstrapping information somewhere centrally, and fetching them at runtime as needed.