Google provider accepts additional authentication params

[Jamedjo]

Allows setting config.google.auth_params = {access_type: :offline, hd: 'mydomain.com'}
Solves #702

A better solution might be to merge these params with the ones hard coded in oauth2.rb and passed to client.auth_code.authorize_url. That would then make it easier to extend this to other providers, e.g. to add include_email for #719. I've instead used the auth_url to avoid changing the Sorcery::Protocols::Oauth2#authorize_url method signature for everyone. If you prefer I'll add an extra argument there instead, i.e. authorize_url(options = {}, auth_params = {}).