nwggrid: allow using pins & favs together with -d ... argument

[Siborgium]

The main problem is, we'd have to maintain separate caches for each custom directory, otherwise the main cache is going to be messed up. While it's not the most important thing to do right now, this feature would be nice to have.

Even with 7f07946 it's not immediately clear what's happening and why it's happening (see #183), that's why implementing it should be a good thing.