Add Microsoft OAuth2 library to vendor.

vendor/golang.org/x/oauth2/LICENSE

-Copyright (c) 2009 The Go Authors. All rights reserved.
+Copyright (c) 2009 The oauth2 Authors. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are

vendor/golang.org/x/oauth2/microsoft/microsoft.go

+// Copyright 2016 The Go 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 microsoft provides constants for using OAuth2 to access Windows Live ID.
+package microsoft // import "golang.org/x/oauth2/microsoft"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// LiveConnectEndpoint is Windows's Live ID OAuth 2.0 endpoint.
+var LiveConnectEndpoint = oauth2.Endpoint{
+	AuthURL:  "https://login.live.com/oauth20_authorize.srf",
+	TokenURL: "https://login.live.com/oauth20_token.srf",
+}

vendor/vendor.json

@@ -878,6 +878,12 @@
 			"revision": "bb50c06baba3d0c76f9d125c0719093e315b5b44",
 			"revisionTime": "2017-09-28T00:25:42Z"
 		},
+		{
+			"checksumSHA1": "aoeqnmNuZdqxHG0/8bhKNLfRaY4=",
+			"path": "golang.org/x/oauth2/microsoft",
+			"revision": "b9780ec78894ab900c062d58ee3076cd9b2a4501",
+			"revisionTime": "2017-02-14T22:24:16Z"
+		},
 		{
 			"checksumSHA1": "FUo16zuO20jOcdCUu0IgiF51koE=",
 			"path": "golang.org/x/sys/unix",