title: issue with custom validation rule for duplicate entry in array
body: <p>I am trying to use knockout.validation plugin. I want created a custom validation rule for checking the duplicate array item while making entry in the array. I tried this :</p>
  
  <pre><code>ko.validation.rules['duplicate'] = {
      validator: function (val1, val2) {
  
      },
      message: 'The value is already exist'
  };
  
  ko.validation.registerExtenders();
  </code></pre>
  
  <p>Here <code>val1</code> is the array and <code>val2</code> is the parameter pass in with the extend method. The problem is, i don`t know how to access the current entered item in the array from custom validation function. help me please ?</p>