Remove old backups (#11)

- Added `delete` script 🎉.
  - `DELETE_AFTER` variable must be greater than `0`.

delete.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+# Go to the backups directory.
+cd /backups
+
+# Delete tar.xz archives older than x days.
+find . -iname "*.tar.xz" -type f -mtime +$DELETE_AFTER -exec rm -f {} \;
+
+# Echo that script ran.
+echo "Deleted files older than $DELETE_AFTER days."