cmd/age: add a prototype of the command line tool

internal/format/format.go

@@ -1,3 +1,9 @@
+// Copyright 2019 Google LLC
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
 package format
 
 import (