title: JsPdf Auto Table : Customize headers with colspan
body: <p>I need to export a few tables to PDF format. I decided to use jspdf-autotable but I can't find a way to customize my header. I need to do this in the header style :</p>
  
  <pre><code> _________________________________________
   |__________|______________|_____________|
   |____|_____|___|__|___|___|___|___|__|__| 
   |____|_____|    ... (content)
   |____|_____|    ... (content)
  </code></pre>
  
  <p>The first line is my problem, as headers are define differently, and I cant find a way to do this through the "drawHeaderCell" method.
  Do u know a way to do this ?</p>
  
  <p>Thank you</p>