Add field keywords to rewrite middleware (#497)

* Require Field for rewrite rules

* review feedback changes

* fix ut

* fix typo, add warning message

middleware/rewrite/field.go

@@ -0,0 +1,13 @@
+// Package rewrite is middleware for rewriting requests internally to something different.
+package rewrite
+
+/*
+Additional FIELD keywords may be implemented to support more rewrite use-cases.
+New Rule types must be added to the Fields map.
+The type must implement `New` and `Rewrite` functions.
+*/
+var Fields = map[string]Rule{
+	"name":  NameRule{},
+	"type":  TypeRule{},
+	"class": ClassRule{},
+}