send explicit pings to client

static/main.html

@@ -8,12 +8,19 @@
     <section id="log">
       <h1>Incoming Events</h1>
     </section>
+    <template id="logline">
+      <p>Ping
+    </template>
     <template id="ring">
       <p>Ring!
     </template>
     <script>
       "use strict";
       let sse = new EventSource('events');
+      sse.addEventListener('ping', msg => {
+        const template = document.querySelector('#logline').content.cloneNode(true);
+        document.querySelector('#log').appendChild(template);
+      });
       sse.addEventListener('ring', msg => {
         const template = document.querySelector('#ring').content.cloneNode(true);
         document.querySelector('#log').appendChild(template);