Whitelist repos

[bj-mcduck]

This is a feature request.
Our organization has a bunch of repos, and we'd like to run peril on a handful of them.
It would probably be nice to have an optional allowed_repos/watched_repos type value for whitelisting which repos to run on rather than having to blacklist each one.

I'm new as of today to peril, so not sure if I'll be able to swing doing this myself yet.
If I can, I'll see if I can get a PR together for it.

[orta]

You probably shouldn't need this because you can install GitHub apps on only specific repos instead of all of them, I'm not particularly against the feature (because then it's user-owned and not admin owned) so you're welcome t ogive it a shot