Add analytics script

Author: jdomingu

docs/_includes/analytics.html

@@ -0,0 +1,7 @@
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+})(window,document,'script','dataLayer','GTM-BVCN');</script>
+<!-- End Google Tag Manager -->

docs/_includes/head.html

@@ -13,3 +13,5 @@
 
   <script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
   <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
+
+{% if jekyll.environment == "production" %}{% include analytics.html %}{% endif %}