Add beginnings of new cr command

go/src/infra/tools/cr/lib/subcommand/subcommand.go

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/*
+Library for defining subcommands in a structured way.
+*/
+
+package subcommand
+
+import (
+        "flag"
+        "fmt"
+)
+
+type Subcommand struct {
+        ShortHelp string
+        LongHelp  string
+        flagFn    func(*flag.FlagSet)
+        runFn     func(*flag.FlagSet) error
+}
+
+func New(shortHelp string, longHelp string, flagFn func(*flag.FlagSet), runFn func(*flag.FlagSet) error) *Subcommand {
+        return &Subcommand{shortHelp, longHelp, flagFn, runFn}
+}
+
+func (c *Subcommand) Help(flags *flag.FlagSet) {
+        fmt.Println(c.ShortHelp)
+        fmt.Println("")
+        fmt.Println(c.LongHelp)
+        fmt.Println("")
+        flags.PrintDefaults()
+}
+
+func (c *Subcommand) InitFlags(flags *flag.FlagSet) {
+        if c.flagFn != nil {
+                c.flagFn(flags)
+        }
+}
+
+func (c *Subcommand) Run(flags *flag.FlagSet) error {
+        if c.runFn != nil {
+                return c.runFn(flags)
+        }
+        return nil
+}