Use base branches as option

[guizoxxv]

I think it would be nice to be able to consider the base branch as option to decide if a issue (PR) should be processed. Right now, the only option I see is to add a label on every PR to achieve this result, which is not very practical.

We could have a only-base-branches to specify that only PRs to those branches should be processed and a exempt-base-branches to specify that PRs to those branches should not be processed.

I quickly checked the code and GitHub API, and one problem I noticed is that the List Repository Issues endpoint that we use, does not have the base branch information available. A request to the Get Pull Request endpoint would be required for every PR (if not skipped by another option). Another option would be to make a request to the List Pull Requests endpoint with the base query parameter filter and then check if the PR is in this list, which would result in less requests.

I'm willing to work on this feature, but I wanted to first check if it makes sense. Thanks.