githubbot: ability to subscribe to only failure statuses

[tchajed]

I'd like to get notifications in Keybase when our continuous integration fails, but it would be a lot of noise to have every success posted. We could have a separate channel for these notifications and filter to successes, but it would be more convenient if we could just subscribe to failures.

Bonus points for being able to also subscribe to newly-passing successes, but this is less important.