website: add google analytics

Author: changkun

website/themes/moderncpp/layout/layout.ejs

@@ -10,6 +10,15 @@
     <link rel="shortcut icon" type="image/x-icon" href="/modern-cpp/assets/cover-2nd.png">
     <meta name="msapplication-TileColor" content="#7e2d36">
     <meta name="theme-color" content="#7e2d36">
+    <!-- Global site tag (gtag.js) - Google Analytics -->
+    <script async src="https://www.googletagmanager.com/gtag/js?id=UA-80889616-2"></script>
+    <script>
+      window.dataLayer = window.dataLayer || [];
+      function gtag(){dataLayer.push(arguments);}
+      gtag('js', new Date());
+
+      gtag('config', 'UA-80889616-2');
+    </script>
 
     <link href='//fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,600|Roboto Mono' rel='stylesheet' type='text/css'>
     <link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" type="text/css">