title: Add a computed property to a select prompt
body: <p>I'd like to render a prompt which reads "All products (500)". 500 being the total number of available products. <code>prompt="All products {{filteredProductsCount}}"</code> doesn't work.</p>
  
  <p>What do I have to change to add the value of <code>filteredProductsCount</code> to the prompt?</p>
  
  <p><strong>controller.js</strong></p>
  
  <pre><code>filteredProductsCount: function() {
    var filteredProducts = this.get('filteredProducts');
    return filteredProducts.length;
  }.property('filteredProducts')
  </code></pre>
  
  <p><strong>template.hbs</strong></p>
  
  <pre><code>{{view "select" prompt="All products"
         content=filteredProducts
         value=selectedProduct
         class="form-control"}}
  </code></pre>