title: Can Angular 5 Service Worker fetch assets in background
body: <p>I'm ramping up on progressive web apps using Angular 5.  We have a CMS that delivers raw JSON for our application.  For content that has images, the JSON contains URLs to the images residing on our server.  I'm able to retrieve from our CMS a list of all images (or a filtered list based on conditions).  </p>
  
  <p>Does it make sense to retrieve that list of image URLs using a fetch command and have Service Worker cache those assets ahead of time?  If so, can Angular 5's implementation of Service Worker be customized to handle a fetch scenario in the background?  Or, do I have to create a manual Service Worker to fetch a list of URLs to image assets then cache them?</p>