Permit a single layout stylesheet for each content type

Eg: /css/post.css

layouts/partials/header.html

@@ -6,6 +6,11 @@
     {{ "<!-- Tumble theme for Hugo: https://github.com/adrianshort/tumble -->" | safeHTML }}
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <link rel="stylesheet" href="{{ relURL "/css/main.css" }}">
+
+    {{- with .Type -}}
+    <link rel="stylesheet" href="{{ relURL "/css/" }}{{ . }}.css">
+    {{ end }}
+
     <link rel="alternate" type="application/rss+xml" title="{{ partial "data/title" . }}"
       href="{{ .RSSLink }}">
     {{- template "_internal/opengraph.html" . -}}