title: How to add a handler
body: <p>I have the following code:</p>
  
  <pre><code>Banner.modules = {
      toolbar: [
          [{ 'header': '1'}, {'header': '2'}, { 'font': [] }],
          [{size: []}],
          ['bold', 'italic', 'underline', 'strike', 'blockquote'],
          [{'list': 'ordered'}, {'list': 'bullet'},
              {'indent': '-1'}, {'indent': '+1'}],
          ['link', 'image', 'video'],
          ['clean']
      ],
  
      clipboard: {
          // toggle to add extra line breaks when pasting HTML:
          matchVisual: false,
      }
  }
  </code></pre>
  
  <p>How can I add a custom handler for <code>image</code>? (when an image is uploaded)</p>