Allow cached resources

[bahmutov]

Like JS and CSS. Probably pass a list into the call to setup.
The fetch event should become something like

const urls = ['/', 'app.css']
  self.addEventListener('install', (event) => {
    event.waitUntil(
      caches.open('demo').then((cache) => cache.addAll(urls))
    )
  })

  self.addEventListener('fetch', function (event) {
    const parsedUrl = url.parse(event.request.url)
    console.log(myName, 'fetching page', parsedUrl.path)

    if (isJsRequest(parsedUrl.path) || isCssRequest(parsedUrl.path)) {
      event.respondWith(
        caches.open('demo').then(cache => {
          return cache.match(event.request)
            .then(cached => {
              if (cached) {
                return cached
              }
              return Promise.reject()
            })
          .catch(err => fetch(event.request))
        })
      )
      return