Make `mime-db` a peer-dependency?

[Robert Kieffer]

'Not sure if this makes sense, but it'd be nice if this module could get out of the business of having to update the mime-db dependency every time there was a new release there.

Would probably require users to run npm install mime-db mime. This would also require re-running the build.js script anytime that dependency was changed or updated, which could get problematic.