title: How to pre-populate email in the Stripe payments dialog popup
body: <p>I cannot seem to find a way to pre-populate the email address in the stripe payment popup. However this weekend I signed up for two accounts on websites that use stripe payments and I realized those websites had my email pre-populated in the stripe dialog iframe box. So I know there must be a way but I am unsure of how to do that. The docs don't define that property. 
  Can someone explain how this is done using the javascript API and the basic Stripe dialog?</p>