Add option to apply a filter to an AssetCollection only after all inner assets are dumped

I was trying to make a gzip filter for js and css filies, using the system command gzip. All worked well dumping the assets when the assets arn't combined.

But if you try to combine several assets into a single file the problem arrives since all files are gzipped together and than joined togheter with \n. So the resulting file is not valid. It would be greet if a filter could be applied just the moment right before dumping it to disk, and I think it would not be just great for this usecase, but allso for minifying and such.