v0.1.x supported an extra JSON key 'authType', which corresponds to the auth_type parameter in the authorization redirect.
v0.2.0 adds support in OAuthSettings.userAuthUri for passing extra parameters to the authorization endpoint. This is now the preferred way to pass the auth_type parameter when needed. Using the JSON key is deprecated.
See OAuthSettings constructor for common documentation.
The 'provider' field may be omitted. If it is included, it MUST be set to "facebook".