* middleware/httproxy: implement debug queries
Not too useful at the moment, but o-o.debug queries are supported
and return the Comment from dns.google.com.
Note that this is not always set.
* improve documentation
* Testing cleanups
This PR adds the *root* middleware that specifies a path where
all zone file (the *file* middleware is the only consumer now) can
be found. It works the same as in Caddy.
Documentation can be found in the README.md of the middleware.
Fixes#307