title: Custom IPrincipal together with WindowsAuthentication
body: <p>Is there any good way of combining ASP.NET Windows Authentication with a custom IPrincipal/IIdentity object? I need to store the user's email address and have done so for Forms Authentication using a custom IIdentity/IPrincipal pair that I added to the Context.CurrentUser during the AuthenticateRequest event. </p>
  
  <p>How would I best go by to accomplish this using WindowsAuthentication?</p>