Running the example:
base_url=https://gitlab.your-host client_secret=$client_secret client
_id=$client_id port=8081 go run main.go
2018/10/10 12:57:57 Using port: 8081
Now navigate to http://localhost:8081/start/ to begin the flow.
2018/10/10 12:57:58 URL: /oauth2/authorized?custom_param=1&code=xyz&state=state
2018/10/10 12:57:58 map[custom_param:[1] code:[xyz] state:[state]]