title: How to hook into react-final-form updates?
body: <p>I'd like to hook into react-final-form's updates. Is that possible? Namely, I'd like to be able to execute an arbitrary function that will update the data that react-final-form is in charge of. In particular, I have a <code>Model</code> class with an <code>update</code> method, and I'd like to be able to call <code>instance.update({ [fieldName]: fieldValue })</code> instead of updating the plain ol' Javascript object that react-final-form uses. Is this possible?</p>
  
  <p>An example of what I'd like to do is as follows:</p>
  
  <pre class="lang-js prettyprint-override"><code>&lt;Form
    initialValue={myModel} // instance of Model
    render={() =&gt; &lt;SomeField name="height" /&gt;}
    updated={(values, name, value) =&gt; values.update({ [name]: value })}
  /&gt;
  </code></pre>