Merge branch 'add-ga' into 'master'

Add Google Analytics.

See merge request !158

layouts/partials/custom-head.html

 <link rel="stylesheet" href="/css/custom.css">
 <link href="//cloud.typography.com/7717652/616662/css/fonts.css" type="text/css" rel="stylesheet">
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-36141757-1"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'UA-36141757-1');
+</script>
 {{ if isset .Params "scripts" }}{{ range .Params.scripts }}<script src="{{ printf "%s" . }}"></script>{{ end }}{{ end }}
 {{ if isset .Params "css" }}{{ range .Params.css }}<link rel="stylesheet" href="{{ printf "%s" . }}">{{ end }}{{ end }}