Add support for Google Analytics 4

Author: rcvalle

_includes/analytics.html

@@ -9,3 +9,14 @@
   ga('send', 'pageview');
   </script>
 {% endif %}
+{% 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 %}