Currently you can only filter on the target branch, but it'd be nice to be able to filter on the source branch name within the PR config.

E.g. have a PR config with:
- source branch pattern (the new option I'm suggesting)
- target branch pattern (currently the only option)

Then I could open a PR against the stack branch (e.g. staging), but only have it spin up a preview environment if my source branch is e.g. feature-123 and my source pattern is feature*.