Keep original css 'fixes' #50

Ok, so i'm not sure if you want to merge this and add support for the `--keep-original-css` flag.

I do think it's useful to be able to always keep the css exactly as it was.

If that was the intention of the `--preserve-style-tags` then that's a bug as it doesn't work for the test file `test-issue50.html`

Basically when there are media queries included it breaks.

So, this really doesn't fix the bug and if the intention of --preserve-style-tags is to always keep all of the css (including comments for example) then I would suggest doing as `--keep-original-css` is doing it here.

Let me know what you want to do.