title: validation event to fire when duplicate chips not added
body: <p>When I try to add duplicate chips the component automatically delete the new added chip as it doesn't allow adding duplicated chips.</p>
  
  <p>I need to fire an event which I can depend on to view a validation message for the user that he can't add duplicate chips</p>
  
  <hr>
  
  <pre><code>&lt;div ng-controller="BasicDemoCtrl as ctrl" layout="column"
       ng-cloak="" class="chipsdemoBasicUsage" ng-app="MyApp"&gt;
  
    &lt;md-content class="md-padding" layout="column"&gt;
  
       &lt;h2 class="md-title"&gt;Make chips editable.&lt;/h2&gt;
  
         &lt;md-chips ng-model="ctrl.editableFruitNames" readonly="ctrl.readonly"
                   md-removable="ctrl.removable" md-enable-chip-edit="true"
                   input-aria-label="Fruit names"&gt;
         &lt;/md-chips&gt;
         &lt;em ng-if="if chips duplicated(the condition which I need)"&gt;You have 
          added a duplicated chip&lt;/em&gt;
  </code></pre>
  
  <p>try to add duplicate chips .. the duplicated one will be automaticlly deleted without any validation message <a href="https://codepen.io/aRagabCs/pen/OYXwEx" rel="nofollow noreferrer">https://codepen.io/aRagabCs/pen/OYXwEx</a></p>