twitch: add twitch OAuth2 endpoint

Twitch Auth API documentation: https://dev.twitch.tv/docs/authentication

Change-Id: I1a517609c229c8893d2dbf91f8821db386f822ab
Reviewed-on: https://go-review.googlesource.com/83136
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: while-loop

twitch/twitch.go

@@ -0,0 +1,19 @@
+// 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 twitch provides constants for using OAuth2 to access Twitch.
+package twitch // import "golang.org/x/oauth2/twitch"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Twitch's OAuth 2.0 endpoint.
+//
+// For more information see:
+// https://dev.twitch.tv/docs/authentication
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://api.twitch.tv/kraken/oauth2/authorize",
+	TokenURL: "https://api.twitch.tv/kraken/oauth2/token",
+}