add ability to set extra params to oauth2 url endpoints

OAuth2 providers [must](http://tools.ietf.org/html/rfc6749#section-3.1) [ignore](http://tools.ietf.org/html/rfc6749#section-3.2) non-oauth2 request parameters to oauth2 endpoints, and some providers (e.g. Google) do use extra parameters to alter the flow in various ways.

This patch provides a way to configure extra parameters to be sent to oauth2 endpoints.  It may also help resolve issue #158.