oauth2: add Yandex endpoint

Checked with simple application, worked good for me.

Change-Id: I068b33c1b44a338a7b0a9b17220bc39db81f2eed
Reviewed-on: https://go-review.googlesource.com/37443
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: dartkron

yandex/yandex.go

@@ -0,0 +1,16 @@
+// Copyright 2017 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 yandex provides constants for using OAuth2 to access Yandex APIs.
+package yandex // import "golang.org/x/oauth2/yandex"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is the Yandex OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://oauth.yandex.com/authorize",
+	TokenURL: "https://oauth.yandex.com/token",
+}