title: Reverse polarity of an angular.js filter
body: <p>Given a filter method that returns true if some condition is met, is it possible to invoke its opposite in HTML, e.g. to use:</p>
  
  <pre><code>"item in items | filter:!AllDay"
  </code></pre>
  
  <p>instead of  </p>
  
  <pre><code>"item in items | filter:AllDay"
  </code></pre>
  
  <p>? Or do you have to maintain two separate filter methods (one for false and one for true)? </p>