title: jspdf Autotable with colspan for last some rows
body: <p>This is actually 3 column table, I need last three rows should be two columns, that means colspan 2 ,
  How can we use the colspan for some rows ?
  This is the table structure and the code</p>
  
  <pre><code>--------------------------------
  | Si     |  item     |  Amount |
  --------------------------------
  |  1     | Keyboard  |  10 $   |
  |  2     | Mouse     |  5  $   |
  --------------------------------
  
  
  this.cols.push({si:"1",itm:"keyboard",amt:"10"})
  this.cols.push({si:"2",itm:"Mouse",amt:"5"})
  var columns = [
                  {title: "Si.", dataKey: "si"},
                  {title: "Item", dataKey: "itm"},
                  {title: "Amount", dataKey: "amt"},
  
              ]
  var a = doc.autoTable(columns, this.cols, {
                  theme: 'striped',
                  styles: {
                      lineWidth: 0.2,
                      lineColor: 0,
                      fontSize:8
                  },
                  headerStyles: {
                      fillColor: 230,
                      textColor:0,
                      fontStyle:'normal', 
                       halign: 'center'           
  
                  },
                  margin: {top: 92},
                  columnStyles:{
                      value_usd: {halign: 'right'},
                      value_aed: {halign: 'right'}
                  }
              });
  </code></pre>
  
  <p>The result expected:</p>
  
  <pre><code>--------------------------------
  | Si     |  item     |  Amount |
  --------------------------------
  |  1     | Keyboard  |  10 $   |
  |  2     | Mouse     |  5  $   |
  --------------------------------
  | Total              |  15 $   |
  --------------------------------
  | VAT                |  5%     |
  --------------------------------
  | Grand Sum          |  15.75  |
  </code></pre>