title: Set dropdown input default value based on third parameter in Grocery CRUD
body: <p>Code sample below, </p>
  
  <pre><code>function product($parameter){
  
     $crud = new grocery_CRUD();
     ...
     $crud-&gt;callback_add_field('dropdown_field_name',array($this,'_add_field_callback'));
     ...
     $output = $crud-&gt;render();
  }
  </code></pre>
  
  <p>Can I do something like this ? </p>
  
  <pre><code>function _add_field_callback($parameter){
     //load db model
     //call the result and return as dropdown input field with selected selection when value = $parameter 
  }
  </code></pre>