Add rolling backups module

[3noch]

I've written a lot of functions to set up rolling backups that could be generally useful. I need to turn them into a NixOS module and include them here, or via some other repo.

[3noch]

This is likely a better fit for a separate repo, since backups tend to be very specific to a certain set of backup tools. The trick will be to make this setup configurable enough to allow for a backup system to easily plugin.