Create an issue for Pull Request that is merged, but the branch hasn't been deleted after a period of time

[Stargator]

By watching events for Pull Requests that are merged, Arborist should check after X amount of time that the branch was deleted.

If not, then Arborist opens an issue, listing the branch and explaining why that list exists.

If #4 hasn't been implemented, then use the default configuration values as defined in the README.md to determine how long to wait.