title: Stripe checkout.js - passing custom params to token callback
body: <p>I'm following this example integration from Stripe Docs (slightly modified in order to be able to add click handlers to more than one button:</p>
  
  <pre><code>&lt;script src="https://checkout.stripe.com/checkout.js"&gt;&lt;/script&gt;
  &lt;button id="customButton"&gt;Purchase&lt;/button&gt;
  &lt;script&gt;
    var handler = StripeCheckout.configure({
      key: 'pk_test_jPVRpCB1MLjWu2P71eTvXBZD',
      image: '/square-image.png',
      token: function(token) {
        // Use the token to create the charge with a server-side script.
        // You can access the token ID with `token.id`
      }
    });
  
    $('.pay-deposit').click( function(e) {
      // Open Checkout with further options
      handler.open({
        name: 'Demo Site',
        description: '2 widgets ($20.00)',
        amount: 2000
      });
      e.preventDefault();
    });
  </code></pre>
  
  <p>In my particular case I have a few buttons like:</p>
  
  <pre><code>&lt;button class='pay-deposit' booking-id='3455'&gt;Pay Deposit&lt;/button&gt;
  &lt;button class='pay-deposit' booking-id='335'&gt;Pay Deposit&lt;/button&gt;
  &lt;button class='pay-deposit' booking-id='34'&gt;Pay Deposit&lt;/button&gt;
  </code></pre>
  
  <p>... and obviously I'd like to pass a booking-id of clicked button somehow to token callback. Couldn't find any example or explanation covering this seemingly simple case... any help much appreciated. thanks!</p>