feat(#6): Add PWA functionality and service worker

src/template.html

@@ -26,5 +26,11 @@
         <textarea id="input-lines" rows="15" cols="60" name="text"
             placeholder="Enter names to be shuffled, one per line. Leave this area empty in order to use a default set of generated names."></textarea>
         <button id="startSpinning">Start spinning</button>
+
+        <p id="install-app-btn-container">
+            <button id="install-offline-app" class="btn btn-secondary btn-xs">
+                <span>Install the app (you can even use it offline!)</span>
+            </button>
+        </p>
     </body>
 </html>