feat(response): deal with mime types

Now, we'll reply with the correct mime type depending on the
request file. For this, we'll be using the mime module, and whenever
it fails, we'll use the mime type coming from GitHub, which usually
is text/plain or application/octet-stream, so we don't have to read the
file on our side and try to identify if it's text or binary.

Closes #2
refactor(package.json): use better node engine version

To dismiss another Heroku "pro tip", we'll not rely anymore on
the '>=' operator for the node engine. Now, let's just use the '~'
operator, which does basically the same thing.