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.

middleware/httpproxy/README.md

@@ -0,0 +1,50 @@
+# httpproxy
+
+*httpproxy* proxies DNS request to a proxy using HTTPS (or HTTP/2 - not implemented). Usually this
+ involves sending a JSON payload over this transport and translating the response back to DNS. The
+ current supported backend is Google, using the URL: https://dns.google.com .
+
+## Syntax
+
+In its most basic form, a simple http proxy uses this syntax:
+
+~~~
+httpproxy FROM TO
+~~~
+
+* **FROM** is the base domain to match for the request to be proxied.
+* **TO** is the destination endpoint to proxy to, accepted values here are `dns.google.com`.
+
+For changing the defaults you can use the expanded syntax:
+
+~~~
+proxy FROM TO {
+    upstream ADDRESS...
+}
+~~~
+
+* `upstream` defines upstream resolvers to be used (re-)resolve `dns.google.com` (or other names in the
+  future) every 30 seconds. When not specified the combo 8.8.8.8, 8.8.4.4 is used.
+
+## Metrics
+
+If monitoring is enabled (via the *prometheus* directive) then the following metric is exported:
+
+* coredns_httpproxy_request_count_total{zone, proto, family}
+
+## Examples
+
+Proxy all requests within example.org to Google's dns.google.com.
+
+~~~
+proxy example.org dns.google.com
+~~~
+
+Proxy everything, and re-lookup `dns.google.com` every 30 seconds using the resolvers specified
+in /etc/resolv.conf.
+
+~~~
+proxy . dns.google.com {
+    upstream /etc/resolv.conf
+}
+~~~