add google analytics

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-160173841-20"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-160173841-20');
</script>