Allow cache directory to be configured

[kevinwright]

...and allow for a shared cache directory

By locating the cache under node_modules, a download is forced afresh for every clean build in which this directory is deleted, and multiple nw.js projects are all forced to download their own identical copies of the binaries.

This is insanely wasteful in terms of both drive space and network bandwidth. Can we have an option allowing binaries to be cached in e.g. ~/.nwjs-builder-phoenix/caches, or at least allow users to specify the cache location so that I might configure this myself?