Add field keywords to rewrite middleware (#497)

* Require Field for rewrite rules

* review feedback changes

* fix ut

* fix typo, add warning message

middleware/rewrite/name.go

@@ -0,0 +1,29 @@
+// Package rewrite is middleware for rewriting requests internally to something different.
+package rewrite
+
+import (
+	"strings"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/dns"
+)
+
+// NameRule is a name rewrite rule.
+type NameRule struct {
+	From, To string
+}
+
+// Initializer
+func (rule NameRule) New(args ...string) Rule {
+	from, to := args[0], strings.Join(args[1:], " ")
+	return &NameRule{middleware.Name(from).Normalize(), middleware.Name(to).Normalize()}
+}
+
+// Rewrite rewrites the the current request.
+func (rule NameRule) Rewrite(r *dns.Msg) Result {
+	if rule.From == r.Question[0].Name {
+		r.Question[0].Name = rule.To
+		return RewriteDone
+	}
+	return RewriteIgnored
+}