Support GitHub App with OAuth

[chris-feist]

GitHub allows apps to optionally add OAuth during the app installation, see step 11 here. In this case, your callback URL is called directly after a user installs the app on GitHub.com with an auth code to exchange for a user token. However, this library fails to handle this because there is no state (relevant code). Below example url signature for this scenario.

Please add support for this auth flow!

http://localhost:3000/auth/github/callback?code=01234567890abcdef012&installation_id=01234567&setup_action=install

Debug error in console: OAuth2Strategy State from URL null