Add device auth url to endpoint (#11)

oauth2.go

@@ -76,8 +76,9 @@ type TokenSource interface {
 // Endpoint represents an OAuth 2.0 provider's authorization and token
 // endpoint URLs.
 type Endpoint struct {
-	AuthURL  string
-	TokenURL string
+	AuthURL       string
+	DeviceAuthURL string
+	TokenURL      string
 
 	// AuthStyle optionally specifies how the endpoint wants the
 	// client ID & client secret sent. The zero value means to
@@ -298,7 +299,6 @@ func (tf *tokenRefresher) Token() (*Token, error) {
 		"grant_type":    {"refresh_token"},
 		"refresh_token": {tf.refreshToken},
 	})
-
 	if err != nil {
 		return nil, err
 	}