title: VueJs + Element UI : How to custom el-dialog title?
body: <p>I created a dialog with a custom el-dialog title using the el-icon, but the el-icon does not appear.
  how to make the icon appear ?</p>
  
  <p>here my code</p>
  
  <pre><code>&lt;el-dialog :title="titleData" :visible.sync="dialogVisible" width="30%"&gt;
    &lt;span&gt;This is a message&lt;/span&gt;
    &lt;span slot="footer" class="dialog-footer"&gt;
      &lt;el-button @click="dialogVisible = false"&gt;Cancel&lt;/el-button&gt;
      &lt;el-button type="primary" @click="dialogVisible = false"&gt;Confirm&lt;/el-button&gt;
    &lt;/span&gt;
  &lt;/el-dialog&gt;
  
  methods: {
    setTitle() {
    this.titleData = '&lt;el-icon-info&gt;&lt;/el-icon-info&gt;' + 'Info'
  }
  </code></pre>
  
  <p>here my fiddle: <a href="https://jsfiddle.net/dede402/y93kvew5/6/" rel="nofollow noreferrer">https://jsfiddle.net/dede402/y93kvew5/6/</a></p>