title: Subscribe to observable array for new or removed entry only
body: <p>So yes I can subscribe to an observable array:</p>
  
  <pre><code>vm.myArray = ko.observableArray();
  vm.myArray.subscribe(function(newVal){...});
  </code></pre>
  
  <p>The problem is the <code>newVal</code> passed to the function is the entire array. Is there anyway I can get only the delta part? Say the <strong>added</strong> or <strong>removed</strong> element?</p>