Allow scope in selectors

I'd like to be able to separate commit types by scope. For example, I'd like to be able to include chore(deps) updates in a release, but ignore chore(build-deps). Without support for the scope filter, I have to either use a non-standard commit type or mix types for differnt-kinds of dependency updates.

Correct me if I'm wrong, but it seems like this _might_ magically just start to work if https://github.com/googleapis/release-please/pull/2544 is merged? Or at least it'll be easier to implement after that. 🤷