Support Cache-Control: immutable #142

There is a draft extension added by facebook called Cache-Control: immutable. If a response is served as immutable then that means it will never ever change no matter what. It seems like it would be useful to support this.

Ideally if a response has been marked immutable we can just skip any sort of Etag or freshness handling and just blindly serve the response.

More information here: https://bitsup.blogspot.com/2016/05/cache-control-immutable.html