add service worker

js/index.js

@@ -254,3 +254,8 @@ validatePurpose(document.querySelector("#purpose input"));
 
 // generate qr code on first visit
 regenerate();
+
+if ('serviceWorker' in navigator) {
+  navigator.serviceWorker.register('/js/service-worker.js');
+}
+

js/service-worker.js

@@ -0,0 +1,24 @@
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open('app-cache').then(cache => {
+      return cache.addAll([
+        '/',
+        '/index.html',
+        '/css/bulma.min.css',
+        '/js/index.js',
+        '/js/qr-creator.min.js',
+        '/img/icon-192.png',
+        '/img/icon-512.png'
+      ]);
+    })
+  );
+});
+
+self.addEventListener('fetch', event => {
+  event.respondWith(
+    caches.match(event.request).then(response => {
+      return response || fetch(event.request);
+    })
+  );
+});
+