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/bind.go

@@ -0,0 +1,11 @@
+// Package bind allows binding to a specific interface instead of bind to all of them.
+package bind
+
+import "github.com/mholt/caddy"
+
+func init() {
+	caddy.RegisterPlugin("bind", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setupBind,
+	})
+}