Autogenerate a new release PR by merging the dev/rc branch if it exists?

Currently,
- The bot is good at discovering new releases and generating a PR for it
- According to [CFEP-05](https://github.com/conda-forge/cfep/blob/master/cfep-05.md) there can be a dev or rc branch publishing pre-releases (although the bot is currently not discovering new pre-release versions for us)

I am not suggesting to fix item 2, but am simply wondering if it is possible for the bot to take advantage of the fact that a dev/rc branch exists. Specifically, say a package with `x.y.z.rc1` is released from the rc branch, when the bot sends a PR targeting the master branch for the official release `x.y.z`, it should also merge from the rc branch. The reason is that at this point some necessary changes in the recipe are likely already performed by package maintainers in the rc branch, and it'd be nice to simply copy them over to master instead of having maintainers touch the bot's PR manually for such updates. Just a thought.