Add Google Analytics to demo page

We love the stats!

Author: csswizardry

demo/index.html

@@ -16,6 +16,14 @@
     //# sourceURL=obs.inline.js
   </script>
 
+  <script src="https://www.googletagmanager.com/gtag/js?id=G-DNCL1RY4GT" async crossorigin></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag(){dataLayer.push(arguments);}
+    gtag('js', new Date());
+    gtag('config', 'G-DNCL1RY4GT');
+  </script>
+