Syncing only releases

Backstroke is great for syncing commits from a branch. But in certain situations, it'd be ideal to create backstroke PRs only for upstream releases.

Taking `create-react-app` as an example, I don't really want to have Backstroke create a PR for every other commit on `master`, because the changes may not be ready for production. I'd want to have a PR created whenever a release is cut.

What would be the best approach here?

* Option to sync releases instead of choosing a branch, then use the API to find the latest release during sync.
* Be less tied to what Github deems as a release, instead allowing configuration of an optional commit message regex, eg. `^Publish.*$`. This would be easier to implement but less user friendly and prone to false positives.