Add gtag support for Google Analytics (#374)

_includes/gtag.html

@@ -0,0 +1,11 @@
+{% if site.gtag %}
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.gtag }}"></script>
+    <script>
+          window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '{{ site.gtag }}');
+</script>
+{% endif %}