title: How to use Tablesorter's numberSorter?
body: <p>I need to sort numbers in column so that negative numbers always sorted under positive numbers. Read that it can be done with help of numberSorter property but I don't find any example with it.
  For example wen I sort ascending it sorts like this:<br>
  1<br>
  2<br>
  3<br>
  4<br>
  5<br>
  6<br>
  -1<br>
  -2<br>
  -3<br>
  And when I sort descending it sorts like this:<br>
  6<br>
  5<br>
  4<br>
  3<br>
  2<br>
  1<br>
  -3<br>
  -2<br>
  -1  </p>
  
  <p>Their example:</p>
  
  <pre><code>$(function(){
      $("table").tablesorter({
          numberSorter : function(a, b, direction, maxColumnValue){
              // direction; true = ascending; false = descending
              // maxColumnValue = the maximum value of that column (ignoring its sign)
              return a - b;
          }
      });
  });
  </code></pre>
  
  <p>gives me error: <strong>TypeError: x is undefined</strong></p>
  
  <p>How can I do it?</p>