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

core/dnsserver/zdirectives.go

@@ -5,10 +5,10 @@ package dnsserver
 // Directives are registered in the order they should be
 // executed.
 //
-// Ordering is VERY important. Every middleware will
-// feel the effects of all other middleware below
+// Ordering is VERY important. Every plugin will
+// feel the effects of all other plugin below
 // (after) them during a request, but they must not
-// care what middleware above them are doing.
+// care what plugin above them are doing.
 
 var directives = []string{
 	"tls",