Add `op` alias for `xdg-open`

`op <filename>` now opens the file with its associated desktop
application.

shell/common/directories.sh

@@ -19,3 +19,5 @@ alias 6='cd ~6'
 alias 7='cd ~7'
 alias 8='cd ~8'
 alias 9='cd ~9'
+
+alias op='xdg-open'