adds google analytics code

Author: matthewlisp

_config.yml

@@ -71,7 +71,15 @@ gitalk:
   owner: matthewlisp
 
 # 输入Google Analytics web tracking code
-google_analytics:
+google_analytics: <!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-147239486-1"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'UA-147239486-1');
+</script>
 
 # 输入你的网站域名（如果你没有添加自己的域名，而是直接用GitHub pages分配的域名的话可以省略）
 url: