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/bind/README.md

@@ -0,0 +1,22 @@
+# bind
+
+*bind* overrides the host to which the server should bind.
+
+Normally, the listener binds to the wildcard host. However, you may force the listener to bind to
+another IP instead. This directive accepts only an address, not a port.
+
+## Syntax
+
+~~~ txt
+bind ADDRESS
+~~~
+
+**ADDRESS** is the IP address to bind to.
+
+## Examples
+
+To make your socket accessible only to that machine, bind to IP 127.0.0.1 (localhost):
+
+~~~ txt
+bind 127.0.0.1
+~~~