Added head_custom partial.

layouts/partials/head.html

@@ -33,4 +33,5 @@
     <link rel="stylesheet" href="{{ . | absURL }}">
     {{ end }}
 
+{{- partial "head_custom.html" . }}
 </head>

layouts/partials/head_custom.html

@@ -0,0 +1,4 @@
+<!--
+If you want to include any custom html just before </head>, put it in /layouts/partials/head_custom.html
+Do not put anything in this file - it's only here so that hugo won't throw an error if /layouts/partials/head_custom.html doesn't exist.
+-->