Unable to limit builds to "Only PRs and multiple whitelisted branches"

This has been brought up multiple times in previous issues, without any response from CircleCI.

All we need is to be able to specify additional “allowed” branches beyond just the repo’s default branch. PRs + master is good, but PRs + master + develop + anybranchname would save us a ton of trouble.

Another thought - rather than updating the Project Settings to handle specifying multiple branches, maybe it would be easier to provide “PR filtering” on jobs, similar to branch filtering? Something like:

filters
  prs
    only
      - master

^That example would theoretically only run the job for PRs with a target branch of master. If branch filters and pr filters are both specified, the job should run if either of the conditions are met (not all).