title: angular ng-model don't update on dirty value
body: <p>i would like to know if it's possible to 'prevent' an update of an form field (input, select....) that it's dirty due to user interaction.
  If i update the ng-model primitive (ie: set pippo from 1 to X) the input
  will get updated even if i manually edit the control.</p>
  
  <p>It's possible to prevent that?</p>
  
  <pre><code>demo page:
  
  http://plnkr.co/edit/ClXoS7YVcEDtcApsNpde
  </code></pre>
  
  <p>the input field count UP every X secs.
  if i enter "AAA" i would like that update STOP, because
  the input it's "dirty" due to user interaction.</p>