WIP: Add initial restic support.

I've added a handler and example config file for restic. Since I'll be deploying this via Ansible, I'm not intending to write a helper, etc. But, I figured I'd share what basic progress I have made.

examples/example.restic

@@ -38,7 +38,7 @@ tag =
 keep_last =
 
 # For the last n hours/days/weeks/months/years in which a snapshot was made, keep only the last snapshot for each hour/day/week/month/year.
-# Default is to keep 7 daily backups (when "retention = yes", above).
+# When "retention = yes", above, default is to keep the last 7 backups, and 2 each for daily, weekly, monthly and yearly backups.
 keep_hourly =
 keep_daily =
 keep_weekly =