title: extjs vtype: check value using another value in the same row
body: <p>I have an <code>Ext.grid.Panel</code> and i want to validate user input: there is a field named <code>delete</code> and a field named <code>string</code>. Valid input for <code>delete</code> is a number, which can't be greater than the length of a <code>string</code> field in the same row. I already know how to use <code>vtype</code> so now i have</p>
  
  <pre><code>delete : function(val, field){
              var expr = new RegExp("^[-]?[0-9]*[\.]?[0-9]*$");
              var num = expr.test(val);
              if (!num) return false; //can't be not a number
              else{
                   //have no idea...
              }
          }
  </code></pre>
  
  <p>I have no idea how to access <code>string</code> value for the same row.</p>
  
  <p>Hope it's quite clear. Thanks!</p>