Support yarn-path config option in yarnrc

[markjm]

Fixes #347

[orta]

I merged some upstream changes related to the yarnk lockfile which caused some conflicts, and the changes aren't quite simple enough that I'd feel comfortable using the web editor for the conflicts.

I think this is something we should get merged, so if you'd like to rebase, I'm happy to get it in. I think I also agree with @NMinhNguyen on using resolve too (but I'm not 100%)