Make rules more flexible

[bhelm]

I like the idea of a stand-alone tool to decide about file deletion. But the rules can be more flexible.

Currently the rule "days20" would keep one per day for 20 days. If i want to keep 4 per day, i cant.

Coming from btrfs-sxbackup im used to rules like this:
4d:8/d, 1w:4/d, 2w:daily, 1m:weekly, 3m:2/m, 12m:none
which translates to:
If younger than 4 Days, keep everything
For older than 4 days, keep 8 per Day
For older than 1 Week, keep 4 per Day
For older than 2 Weeks, keep 1 per Day
For older than 1 Month, keep 2 per Month
for older than 12 months, keep none.

As Btrfs-SXBackup is written in python too, maybe those rules can be easily ported over?