Only hijack the response if the filetype matches what we want to transform

It seems that the problem in #1 was caused by indiscriminatory hijacking, and specifically unhijacking of requests, which apparently caused the headers to be flushed, meaning no middleware up the chain could do its job any more.

Either this is a bug in hijackresponse or the scaffold we have been using across all these livestyle middlewares. We've just been lucky not to run into this yet because we haven't run with all settings active per default. But that's exactly what I'm doing in the grunt-livestyle wrapper