title: Get previous value of an observable in subscribe of same observable
body: <p>Is it possible in knockout to get the current value of an observable within a subscription to that observable, before it receives the new value?</p>
  
  <p>Example:</p>
  
  <pre><code>this.myObservable = ko.observable();
  this.myObservable.subscribe(function(newValue){
      //I'd like to get the previous value of 'myObservable' here before it's set to newValue
  });
  </code></pre>