This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.
A Corefile like:
.:1053 {
etcd miek.nl
proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.
The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free