Add `Mime::is_compressible` method

Tracking issue: #274

When https://github.com/http-rs/http-types/issues/272 implemented we should add the `Mime::is_compressible` method which can inform which mime types make sense to compress. This is great for negotiating compression in static file servers, as some formats are already compressed when sending over the wire, and applying another round of compression is only going to cost performance.