feat: Added `hook_head_end.html` to head

This makes it easier for theme users to add elements to the head. A file
located at layouts/partials/hook_head_end.html in the site folder will
override the (empty) `hook_head_end.html` in the theme.

layouts/partials/head.html

@@ -9,4 +9,5 @@
     {{ $title := print .Site.Title " | " .Title }}
     {{ if .IsHome }}{{ $title = .Site.Title }}{{ end }}
     <title>{{ $title }}</title>
+    {{ partial "hook_head_end.html" . }}
 </head>