use cache folder as a stand-in for things not found on registry

[isaacs]

Any time the registry returns a 404 (or can't be connected to), but the .cache folder has the thing being requested, use that, even if the cache is expired.

Furthermore, allow a "parent-cache" config param so that it can see stuff in its parent cache.

This should make stuff like issue #437 a lot less painful. Then anything installed can be bundle-installed, since it'll be in the cache.

[isaacs]

This was already done for exact versions, it seems.

The problem with version ranges is that doing this would mean not favoring the latest tag, and has some other unsavory side effects. Punting on this.