anchor.diff:          "title": "Hello world!",
           "image_url": "http://example.com/example.png",
           "subtitle": "This is an example"
  -      }],
  -      "buttons": [{
  -        "type": "web_url",
  -        "url": "http://example.com",
  -        "title": "Show website"
         }]
       }
     }
  
positive.code:                   <i>merchant_name</i>
                  </td>
                  <td>
                    <strong>string</strong> <span class="tag tag-success">Optional</span>
                    <p>Merchant's name.</p>
                  </td>
                </tr>
  
negative.code:               </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button type. Value must be <code>web_url</code>.</p>
                </td>
              </tr>
              <tr>
                <td>
                  <i>title</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button title</p>
                </td>
              </tr>
              <tr>
                <td>
                  <i>url</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button URL</p>
                </td>
              </tr>
            </tbody>
          </table>
        </div>
  
        <pre>
  {
    "message": {
      "mtype": "button",
      "properties": {
        "type": "web_url",
        "title": "View Example.com",
        "url": "http://example.com"
      }
    }
  }</pre>
  
        <hr class="my-3">
  
        <h4>Postback button</h4>
  
        <p>Compatible with Facebook Messenger's <a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference/postback-button">specification for the postback button</a>.</p>
  
        <div class="card">
          <table class="table mb-0">
            <thead>
              <tr>
                <th>Property</th>
                <th>Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>
                  <i>type</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button type. Value must be <code>postback</code>.</p>
                </td>
              </tr>
              <tr>
                <td>
                  <i>title</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button title</p>
                </td>
              </tr>
            </tbody>
          </table>
        </div>
  
        <pre>
  {
    "message": {
      "mtype": "button",
      "properties": {
        "type": "postback",
        "title": "Yes"
      }
    }
  }</pre>
  
        <hr class="my-3">
  
        <h4>Call button</h4>
  
        <p>Compatible with Facebook Messenger's <a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference/call-button">specification for the call button</a>.</p>
  
        <div class="card">
          <table class="table mb-0">
            <thead>
              <tr>
                <th>Property</th>
                <th>Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>
                  <i>type</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button type. Value must be <code>phone_number</code>.</p>
                </td>
              </tr>
              <tr>
                <td>
                  <i>title</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Button title.</p>
                </td>
              </tr>
              <tr>
                <td>
                  <i>payload</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-default">Optional</span>
                  <p>Phone number.</p>
                  <p>Example: <code>+16505551234</code>.</p>
                </td>
              </tr>
            </tbody>
          </table>
        </div>
  
        <pre>
  {
    "message": {
      "mtype": "button",
      "properties": {
        "type": "phone_number",
        "title": "Call me maybe",
        "payload": "+16505551234"
      }
    }
  }</pre>
  
        <hr class="my-3">
  
        <h4>Share button</h4>
  
        <p>Compatible with Facebook Messenger's <a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference/share-button">specification for the share button</a>.</p>
  
        <div class="callout callout-default">
          <div class="callout-content">
            This feature is coming soon.
          </div>
        </div>
  
        <hr class="my-3">
  
        <h4>Buy button</h4>
  
        <p>Compatible with Facebook Messenger's <a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference/buy-button">specification for the buy button</a>.</p>
  
        <div class="callout callout-default">
          <div class="callout-content">
            This feature is coming soon.
          </div>
        </div>
  
        <hr class="my-3">
  
        <h3>Template</h3>
  
        <p><code>mtype</code> must be <code>template</code>.</p>
  
        <h4>Button template</h4>
  
        <p>Compatible with Facebook Messenger's <a href="https://developers.facebook.com/docs/messenger-platform/send-api-reference/button-template">specification for the button template</a>.</p>
  
        <div class="card">
          <table class="table mb-0">
            <thead>
              <tr>
                <th>Property</th>
                <th>Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>
                  <i>type</i>
                </td>
                <td>
                  <strong>string</strong> <span class="tag tag-success">Required</span>
                  <p>Template type. Value must be <code>button</code>.</p>
                </td>
              </tr>
  