[Feature] More flexible schedule with rotate-backups

[ToshY]

Request

Currently it's only possible to use a cron schedule for defining when snapshots are created. This is nice until you realise that you may require a more complex schedule.

As an example, I would also like to keep X weekly snapshots, keep Y monthly snapshots and Z yearly snapshots (ofcourse without manual intervention).

Possible implementation

Something that can be used to create a more flexible schedule is rotate-backups. This works quite well for things like database backups, so I assume it can be used for this as well.