title: @at-root directive not recognized by VS2013
body: <p>I'm editing an existing Sass project, in the mixin.scss file I find the following:</p>
  
  <pre><code>//elements get appended with "__" and the $name
  @mixin e($name) {
    @at-root &amp;__#{$name} {
      @content;
    }
  }
  </code></pre>
  
  <p>That seems correct to me, but Visual Studio 2013 intellisense and compiler gives me an error:</p>
  
  <pre><code>"at-root is not a valid @ directive"
  </code></pre>