title: add code for event listener for keypress in ckeditor
body: <p>I need to add an event listener for keypress after the CKEditor is loaded.
  The code is something like:<br></p>
  
  <pre><code>CKEDITOR.instances.editor1.document.on('key', function(event) {
      /* instructions   */
  });
  </code></pre>
  
  <p>Any idea where can I add the code for that? In which file or in what way?</p>