Update existing issue for any Pull Requests that are closed, but the branch hasn't been deleted

[Stargator]

This can only be done if #12 is already done.

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

If not, then Arborist updates the existing issue for branches that had Pull Requests that were closed.

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