Create analytics.html (ImmutableWebApps#1)

* Create analytics.html

* Add google_analytics to _config.yml

Author: gconnolly

_config.yml

@@ -1,2 +1,3 @@
 theme: jekyll-theme-primer
 title: Immutable Web Apps
+google_analytics: UA-126263119-1

_includes/analytics.html

@@ -0,0 +1,9 @@
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-126263119-1"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '{{ site.google_analytics }}');
+</script>