Bring back support for 'asciicast:play' window event in embed iframe

app/views/api/asciicasts/show.html.slim

@@ -14,4 +14,12 @@ javascript:
       var h = Math.max(document.body.scrollHeight, document.body.offsetHeight);
       target.postMessage(['asciicast:size', { width: w, height: h }], '*');
     }
+
+    function onMessage(e) {
+      if (e.data == 'asciicast:play' || e.data[0] == 'asciicast:play') {
+        player.play();
+      }
+    }
+
+    window.addEventListener("message", onMessage, false);
   });