stackoverflow: add stackoverflow oauth2 endpoints.

Fixes golang#297

Stackoverflow oauth2 endpoints constant added.
Ref: https://api.stackexchange.com/docs/authentication

Change-Id: I8285a9baa9514c79e8325c40434f62aa1ed4939b
GitHub-Last-Rev: 909997b
GitHub-Pull-Request: golang#298
Reviewed-on: https://go-review.googlesource.com/119995
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: manigandand

internal/token.go

@@ -129,6 +129,7 @@ var brokenAuthHeaderProviders = []string{
 	"https://log.finalsurge.com/oauth/token",
 	"https://multisport.todaysplan.com.au/rest/oauth/access_token",
 	"https://whats.todaysplan.com.au/rest/oauth/access_token",
+	"https://stackoverflow.com/oauth/access_token",
 }
 
 // brokenAuthHeaderDomains lists broken providers that issue dynamic endpoints.

stackoverflow/stackoverflow.go

@@ -0,0 +1,16 @@
+// Copyright 2018 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 stackoverflow provides constants for using OAuth2 to access Stack Overflow.
+package stackoverflow // import "golang.org/x/oauth2/stackoverflow"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Stack Overflow's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://stackoverflow.com/oauth",
+	TokenURL: "https://stackoverflow.com/oauth/access_token",
+}