[Enhancement] Adding cache-writer to improve delta performance

This is leveraging broccoli-caching-writer to prevent unwarranted execution of the write function when the sourceTree has not changed.

`less.toCss` is pretty big offender in terms of performance, so CacheWriter will only ever invoke the `updateCache`, previously `write`, function when needed.
This is an interesting pattern.  For my own curiosity, can anyone explain it since it's the first time I've seen it.