Resolve "Respect all known mime types (or just text/cache-manifest)"

Follow-up on !52 (closed) !53 (closed) (thanks @Jonarod!)

Parse https://github.com/jshttp/mime-db (v1.33.0) into Go at compile-time and build it into the binary so we don't have to rely on the presence of /etc/mime.types (and even when that's present, it lacks entries that mimedb has).

Closes #30 (closed) #87 (closed)