title: How to do multi-column header (multi layer header) in SlickGrid?
body: <p>I am using SlickGrid (<a href="https://github.com/mleibman/SlickGrid" rel="nofollow noreferrer">https://github.com/mleibman/SlickGrid</a>) to display an editable table,
  like this one, the one on the right: <img src="https://i.stack.imgur.com/bzWTT.jpg" alt="header span multiple columns"></p>
  
  <p>But seems currently SlickGrid does not support this,
  how can this be done?</p>