Configuration support for additional attic prune flags: keep_within, keep_hourly, keep_yearly, and prefix.

sample/config

@@ -7,6 +7,11 @@ repository: user@backupserver:sourcehostname.attic
 
 [retention]
 # Retention policy for how many backups to keep in each category.
+# See https://attic-backup.org/usage.html#attic-prune for details.
+#keep_within: 3h
+#keep_hourly: 24
 keep_daily: 7
 keep_weekly: 4
 keep_monthly: 6
+keep_yearly: 1
+#prefix: sourcehostname