Prime cache from options

Pass `cache` and `pkgcache` in as options to share caches when building multiple client files with overlapping dependencies trees.  Dropped a clean build of 5 files depending on ~1600 modules from 30s down to 13s.

I'll integrate with grunt-watchify and or grunt-browserify to share the cache by default when you configure a target with multiple destination files.

Could be used to persist cache to disk to speed up subsequent runs when we're confident the files haven't changed.