Add build step to fetch mime.types from Apache and build types.json

Finally getting around to addressing #61.  This also uses `request` to fetch Apache's mime.types file from the source URL, rather than keeping a possibly out-of-date version in this repo, which has caused confusion for people wanting to add new types.  (Of course, I'm sure those same people will end up editing `types.json`, but that's an issue for another day.)