title: Wordpress wp.media Featured Image ID
body: <p>How can I hook into an existing wp.media object and grab the attachment ID when the "Set Featured Image" button is clicked?</p>
  
  <p>The wp.media tutorials I've been looking at all seem to start by creating a new wp.media frame, but I just want to listen for events coming from an existing one (rendered by wp_editor() function), particularly the "Set Featured Image" event.</p>