Make sure that the CachingWriter constructor is called.

Will be needed in order to be able to take advantage of recent/upcoming changes to broccoli-caching-writer (like https://github.com/rwjblue/broccoli-caching-writer/pull/11).
Ignore `filterFromCache` from the command line args.
Merge pull request #28 from timmfin/call-caching-writer-constructor

Make sure that the CachingWriter constructor is called.