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/rewrite/name.go

@@ -0,0 +1,24 @@
+package rewrite
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/miekg/dns"
+)
+
+type nameRule struct {
+	From, To string
+}
+
+func newNameRule(from, to string) (Rule, error) {
+	return &nameRule{plugin.Name(from).Normalize(), plugin.Name(to).Normalize()}, nil
+}
+
+// Rewrite rewrites the the current request.
+func (rule *nameRule) Rewrite(w dns.ResponseWriter, r *dns.Msg) Result {
+	if rule.From == r.Question[0].Name {
+		r.Question[0].Name = rule.To
+		return RewriteDone
+	}
+	return RewriteIgnored
+}