middleware/whoami: add (#264)

Add a new middleware that tells you who you are; IP, port and transport
is echoed back.

Also some various cleanup and documentation improvements while at it:

* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.

middleware/whoami/README.md

@@ -0,0 +1,38 @@
+# whoami
+
+whoami returns your local IP address, port and transport used. Your local IP address is returned in
+the additional section as either an A or AAAA record.
+
+The port and transport are included in the additional section as a SRV record, transport can be
+"tcp" or "udp".
+
+~~~ txt
+._<transport>.qname. 0 IN SRV 0 0 <port> .
+~~~
+
+The *whoami* middleware will respond to every A or AAAA query, regardless of the query name.
+
+## Syntax
+
+~~~ txt
+whoami
+~~~
+
+## Examples
+
+~~~ txt
+.:53 {
+    whoami
+}
+~~~
+
+When queried for "example.org A", CoreDNS will respond with:
+
+~~~ txt
+;; QUESTION SECTION:
+;example.org.   IN       A
+
+;; ADDITIONAL SECTION:
+example.org.            0       IN      A       10.240.0.1
+_udp.example.org.       0       IN      SRV     0 0 40212
+~~~