Add the inevitable Google Analytics tracking

config.toml

@@ -11,3 +11,4 @@ logo_url = "//worldwidetelescope.org/home"
 github_url = "//github.com/WorldWideTelescope/worldwide-telescope-docs-hub"
 twitter_url = "https://twitter.com/wwtelescope"
 facebook_url = "https://facebook.com/wwtelescope"
+google_analytics_id = "UA-107473046-3"

templates/base.html

@@ -35,6 +35,17 @@
     <meta name="msapplication-TileImage" content="ms-icon-144x144.png">
     <meta name="theme-color" content="#ffffff">
     <!-- end of favicons -->
+
+    {% if config.extra.google_analytics_id %}
+    <script async src="https://www.googletagmanager.com/gtag/js?id={{ config.extra.google_analytics_id }}"></script>
+    <script>
+      window.dataLayer = window.dataLayer || [];
+      function gtag(){dataLayer.push(arguments);}
+      gtag('js', new Date());
+      gtag('config', '{{ config.extra.google_analytics_id }}');
+    </script>
+    {% endif %}
+
     <link rel="stylesheet" href="{{ rel_top ~ 'assets/fontawesome.min.css' | safe }}">
     <link rel="stylesheet" href="{{ rel_top ~ 'style.css' | safe }}">
     <!--[if IE]> <script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script><![endif]-->