nixos: add Tarsnap backup service module

This adds a simple module for Tarsnap backups, based on cron and systemd. It creates a systemd service tuned to the specified config, and just uses cron to periodically launch the service. This also makes it trivial to launch backups on demand.

At the moment it defaults to doing backups once per day of the specified directories using an archive name of `nixos-<TIMESTAMP>`. The prefix should probably be configurable I suppose, which I might add soon.