Allow configurable Chef OAuth2 URL

This allows you to set CHEF_OAUTH2_URL to an alternate URL if you're authenticating against a service other than https://id.opscode.com.

The `client_options` given are merged with the ones in [the omniauth-chef-oauth2 configuration](https://github.com/opscode/omniauth-chef-oauth2/blob/9a881294c8e8bcba4f0293eb73fc08d1067bd80b/lib/omniauth/strategies/chef_oauth2.rb#L9-L13).

Leaving this undocumented for now, as we'll be adding a section to the README or another doc on how to use this with your private Supermarket once that's more feasible.