Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/whoami/README.md

@@ -0,0 +1,44 @@
+# whoami
+
+*whoami* returns your resolver's local IP address, port and transport. Your IP address is returned
+ in the additional section as either an A or AAAA record.
+
+The reply always has an empty answer section. 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> .
+~~~
+
+If CoreDNS can't find a Corefile on startup this is the *default* plugin that gets loaded. As
+such it can be used to check that CoreDNS is responding to queries. Other than that this plugin
+is of limited use in production.
+
+The *whoami* plugin will respond to every A or AAAA query, regardless of the query name.
+
+## Syntax
+
+~~~ txt
+whoami
+~~~
+
+## Examples
+
+Start a server on the default port and load the *whoami* plugin.
+
+~~~ corefile
+. {
+    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
+~~~