add enhanced link attribution for google analytics

Author: jontsai

_includes/themes/htk/fragments/js/analytics/google.html

@@ -5,6 +5,7 @@
 m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
 })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 ga('create', '{{ site.analytics.google.tracking_id }}', '{{ site.analytics.google.domain }}');
+ga('require', 'linkid', 'linkid.js');
 ga('require', 'displayfeatures');
 ga('send', 'pageview');
 </script>