title: Mapbox GL Popup set content with custom tag
body: <p>im trying to creater a marker with popup on click, so far so good,
  <strong>the problem is</strong> when im trying to set the content of the popup to be my custom tag,  for example</p>
  
  <pre><code>let popup = new mapboxgl.Popup()
      .setHTML("&lt;custom-tag&gt;&lt;/custom-tag&gt;") 
  </code></pre>
  
  <p>I know about the option of <strong>setDOMContent</strong> but I didn't manage to get it right... it suppose to work with <em>document.createElement('custom-tag')</em> so if you can help me on how to use it with custom components.
  thank you for your help!</p>