Add the ability to set custom fetch attributes as an option on the OpenID strategy

This adds a new attribute "extensions" on the options object that is passed to the OpenID strategy upon initialization.  This new attribute is a key/value pair of additional OpenID attributes (see the Attribute Exchange spec) that will be requested from the OpenID provider.  The key is the URL of the attribute, e.g. "http://schema.openid.net/contact/email", and the value is either "required" or "optional", which instructs the OpenID provider on how to respond if it cannot deliver the information.