Ability to reference files renamed by staticfiles CachedFilesMixin to allow versioning of files via filename

I added a setting called CSS_INVALIDATION_METHOD that allows you to choose an alternative to adding a suffix of a hash of the files contents in compressed files.

The method that I added will rename files by adding a hash of their content to the filename, as the CachedFilesMixin from Django Staticfiles does.  The method renames files via the hashed_name method of the STATICFILES_STORAGE class used so the method of hashing can change.