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/chaos/README.md

@@ -0,0 +1,46 @@
+# chaos
+
+The *chaos* plugin allows CoreDNS to respond to TXT queries in the CH class.
+
+This is useful for retrieving version or author information from the server.
+
+## Syntax
+
+~~~
+chaos [VERSION] [AUTHORS...]
+~~~
+
+* **VERSION** is the version to return. Defaults to `CoreDNS-<version>`, if not set.
+* **AUTHORS** is what authors to return. No default.
+
+Note that you have to make sure that this plugin will get actual queries for the
+following zones: `version.bind`, `version.server`, `authors.bind`, `hostname.bind` and
+`id.server`.
+
+## Examples
+
+Specify all the zones in full.
+
+~~~ corefile
+version.bind version.server authors.bind hostname.bind id.server {
+    chaos CoreDNS-001 info@coredns.io
+}
+~~~
+
+Or just default to `.`:
+
+~~~ corefile
+.  {
+    chaos CoreDNS-001 info@coredns.io
+}
+~~~
+
+And test with `dig`:
+
+~~~ txt
+% dig @localhost CH TXT version.bind
+...
+;; ANSWER SECTION:
+version.bind.		0	CH	TXT	"CoreDNS-001"
+...
+~~~