Add beginnings of new cr command

go/src/infra/tools/cr/cmd/firstrun/firstrun.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.
+
+// Package firstrun contains the logic for the 'cr firstrun' command.
+// Generally only executed once on a given machine, the firstrun command
+// installs the cr tool, prepares a space for it to install modules, and
+// adds it to $PATH.
+package firstrun
+
+import (
+        "flag"
+
+        "infra/tools/cr/lib/subcommand"
+)
+
+var (
+        // Package-level variables for subcommand flags.
+        path string
+)
+
+var shortHelp = "Install the cr tool and intelligently add it to $PATH."
+
+var longHelp = `Given a path to a desired install directory, it takes ownership of that
+directory, sets it up to support installation of modules, and adds the
+directory to $PATH (by prompting to modify e.g. the registry or .bashrc).
+
+The firstrun command is intended to only be run once, when first
+installing the cr tool.
+
+Examples:
+  cr firstrun -verbose -path ~/local/bin/`
+
+func flags(flags *flag.FlagSet) {
+        flags.StringVar(&path, "path", "", "the path to install 'cr' in")
+}
+
+func run(flags *flag.FlagSet) error {
+        firstrunCheckNotInstalled()

[seanmccullough1, 2016/05/06 00:43:54]

What do you think of having stubbable funcs return errors (if only to indicate
they aren't implemented yet for a particular build)?

ex: https://golang.org/src/os/user/lookup_stubs.go

I don't know if _stubs.go is an established pattern, but returning errors at
least seems like a good idea in this situation.

[agable, 2016/05/10 00:34:10]

Yep, great idea. They now return errors.

+        return nil
+}
+
+// FirstrunCmd is a subcommand.Command representing the 'cr firstrun' command.
+var FirstrunCmd = subcommand.New("firstrun", shortHelp, longHelp, flags, run)