Google analytics

Adds Google Tag initialization code for index.html.

<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-JL65GWRNQQ"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'G-JL65GWRNQQQ');
</script>

You should replace the id value with your own.