title: Quill link handler not working
body: <p>I'm trying to write a custom handler for the link input value. In case the user inputs a link that does not have a custom protocol, I wish to prepend a <code>http:</code> before the input value. That's because if link value lacks http:, link is not interpreted and <code>about:blank</code> is shown intead. (<a href="https://github.com/quilljs/quill/issues/1268#issuecomment-272959998" rel="noreferrer">https://github.com/quilljs/quill/issues/1268#issuecomment-272959998</a>)</p>
  
  <p>Here's what I've written (similar to the official example <a href="http://quilljs.com/docs/modules/toolbar/#handlers" rel="noreferrer">here</a>):</p>
  
  <pre><code>toolbar.addHandler("link", function sanitizeLinkInput(linkValueInput) {
      console.log(linkValueInput); // debugging
  
      if (linkValueInput === "")
          this.quill.format(false);
  
      // do nothing, since it implies user has just clicked the icon
      // for link, hasn't entered url yet
      else if (linkValueInput == true);
  
      // do nothing, since this implies user's already using a custom protocol
      else if (/^\w+:/.test(linkValueInput));
  
      else if (!/^https?:/.test(linkValueInput)) {
          linkValueInput = "http:" + linkValueInput;
          this.quill.format("link", linkValueInput);
      }
  });
  </code></pre>
  
  <p>Every time the user clicks the link icon, nothing happens and <code>true</code> is logged to the console. I actually wished this handler to be executed when person clicks "save" on the tooltip that's shown after pressing the link icon.</p>
  
  <p>Any idea how to do this? Hints or suggestions are also appreciated.</p>
  
  <p>Thanks!</p>