title: Modify iron-form JSON before submitting
body: <p>I'm wondering if it is possible to edit the JSON file of an iron-form before submitting it? For instance, if I want to add an array which doesn't come from any of the form input, or if I want to add a unique key in it...</p>
  
  <p>If it is possible, I believe it would be during the form pre-submit, but the documentation says nothing about "how to intercept the JSON" or something like that.</p>
  
  <p>Thanks!</p>