Support ignoring WIP pull requests

If the pull assigner is enabled, then it starts tracking all pull requests and drops a warning comment on all of them when they hit the grace period. It's not a necessity for all PRs to be closed/merged during the given period. So, we should support checking their titles for specific keywords like "WIP" or "Do not merge" (case insensitive) and ignore those PRs, so that they can live beyond the grace period.