Remove non-existing Repos from Backup Folder, --delete

[janmattfeld]

If I remove a repository from GitHub, it might be desirable to remove its files from my backups as well. Usually these repos have no value anymore, or they are stored elsewhere.

So, similar to the rsync --delete argument, not (anymore) existent repositories could be removed from the backup folder.

[josegonzalez]

This isn't something directly useful to me, so pull requests welcome!