title: Knockout - Subscribing to deep observable changes
body: <p>I've got a Knockout application with a hierarchy of objects. If I want to access a deep level in an object in an HTML tag, I'd use something like:</p>
  
  <pre><code>data-bind="text: property1().property2().property3()"
  </code></pre>
  
  <p>I want to subscribe to the same target in pure Javascript i.e. execute a function whenever property3 changes on the currently bound object, or whenever the hierarchy changes below property3. In other words I'd like to do something like:</p>
  
  <pre><code>myviewmodel.deepSubscribe("property1().property2().property3()", function() { .... });
  </code></pre>
  
  <p>How do I go about this in knockout?</p>