title: Google Plus One Button - How to add a callback?
body: <p>I have the google+ button being rendered on my site as follows:</p>
  
  <p>In the JS: </p>
  
  <pre><code>    var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true;
      po.src = 'https://apis.google.com/js/plusone.js';
      var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s);
  </code></pre>
  
  <p>In the html:</p>
  
  <pre><code>&lt;g:plusone size="medium" href="https://site.com"&gt;&lt;/g:plusone&gt;
  </code></pre>
  
  <p>How can I add a callback to this to detect when the user has clicked? I see the docs here: <a href="https://developers.google.com/+/plugins/+1button/#jsapi" rel="noreferrer">https://developers.google.com/+/plugins/+1button/#jsapi</a> not seeing how to implement with the current render. Thanks</p>