Add support for remote cleanup

[andrelopez]

[nitech]

Yes, this would be nice. ~~Also it would be nice to have a built in scheduler that would schedule backing up and cleaning up old backups, ie:~~

It's great that the backup command is callable via WebHooks. It would be great to be able to call the remote cleanup command externally as well.

Clean up old backups: Yes Remote trigger url: [url]

Number of hourly backups to keep: x Number of weekly backups to keep: x Monthly: x Yearly: x

[CreateSean]

Yes, please set up some way to auto-prune backups.