Allowing gzip compression to be disabled by setting gzip: false

This pull request allows gzip compression to be skipped by setting a `gzip` option to `false`

### Why I needed this
My app is based around Restify, which provides its own `gzipResponse` middleware. This middleware gzips all responses blindly. Restify can also serve static content, but its static content serving is very primitive. I wanted to use st for static content, but still allow `gzipResponse` to gzip everything. Without this setting, static content would get gzipped twice.

It is simpler to set up my static endpoints and hand them off to st, than have gzipResponse try and figure out when it should or should not compress.



I'm not sure if this fits your vision for st, but I figured I send a pull request in case it does.