Expand module to permit cron based backups.

[rtprio]

I like the module but on half my systems they don't run systemd. I was adapting it to fallback to cron if the systemd fact doesn't exist but had a couple of questions before I got too far.

First, is this an enhancement that you would be interested in merging?
What I'm trying to work out is the cleanest way to pass both cron related items (hour, minute,weekday) and the existing systemd items (backup_timer).

It would be really nice to have a single time format which can be converted to either but https://man.archlinux.org/man/systemd.time.7 makes that sound extremely complex.