middleware/httpproxy: Add (#439)

This PR adds a middleware that talks to dns.google.com over HTTPS,
meaning all your DNS traffic is encrypted when traversing your ISP and
the internet.

The `dns.google.com` address is re-resolved every 30 seconds.

core/dnsserver/directives.go

@@ -94,5 +94,6 @@ var directives = []string{
 	"etcd",
 	"kubernetes",
 	"proxy",
+	"httpproxy",
 	"whoami",
 }