Provide a way to restrict suites to running only if some file paths changed

[jlebon]

E.g. in ostreedev/ostree#700, we're just updating README.md. We should allow maintainers to define opt-out/opt-in file paths in the YAML for which a specific suite should or shouldn't run.

[mwleeds]

It might make sense to also have a magic phrase you could put in the commit message. For example ostreedev/ostree#1813 is a trivial modification to a path that would otherwise need to be tested.