added 'spa-asset-test' config option. Fixes #36

example/spa/index.html

@@ -7,6 +7,8 @@
   <li><a href="/login">/login</a></li>
   <li><a href="/search">/search</a></li>
 </ul>
+<p>Found asset: <img src="image.jpg" /></p>
+<p>Missing asset (should 404): <img src="sdafsadf.jpg" /></p>
 <script>
   document.querySelector('h2 span').textContent = window.location.pathname
 </script>