Add middleware chaining for request and response modifiers

proxy/modify.go

@@ -7,5 +7,18 @@ var (
 	nopResModifier = func(res *http.Response) error { return nil }
 )
 
+// RequestModifyFunc defines a type for a function that can modify a HTTP
+// request before it's proxied.
 type RequestModifyFunc func(req *http.Request)
+
+// RequestModifyMiddleware defines a type for chaining request modifier
+// middleware.
+type RequestModifyMiddleware func(next RequestModifyFunc) RequestModifyFunc
+
+// ResponseModifyFunc defines a type for a function that can modify a HTTP
+// response before it's written back to the client.
 type ResponseModifyFunc func(res *http.Response) error
+
+// ResponseModifyMiddleware defines a type for chaining response modifier
+// middleware.
+type ResponseModifyMiddleware func(ResponseModifyFunc) ResponseModifyFunc