Use event attribute for funnels #109

Right now, you can only join funnel events on the profile or session. Could it also be possible to join on an attribute? For example, let's say that I have an email funnel with events Email Sent, Email Delivered, Email Opened. Each event has an email_id property. If I send 10 emails to the same user and they only complete the funnel for one of the emails, it looks like the conversion rate is 100% even though it's really only 10%.