[Feature] More flexible schedule with rotate-backups

**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](https://pypi.org/project/rotate-backups/). This works quite well for things like database backups, so I assume it can be used for this as well.