Media type for Accept header parsing

This doesn't fully solve the problem of complex Accept headers (they can be priority lists, and something similar to cowlib's `cow_http_hd.parse_accept` should be used in the future). I will be happy to help out with this time permitting.

This PR also upgrades Plug requirement to 0.9.0 (0.8.4 doesn't seem to have `media_type/1`)