Support categories

Fixes #31

layouts/_default/summary.html

@@ -1,5 +1,9 @@
 <div class="post">
     <h2 class="post-title">
+        {{ range .Params.categories }}
+        <code>{{ . }}</code>
+        {{ end }}
+
         {{ with .Parent.Params.icon }}{{ partial "icon.html" . }}{{ end }}
         <a href="{{ .Permalink }}">{{ .Title | markdownify }}</a>
         {{ if .Draft }}{{ partial "icon.html" "edit" }}{{ end }}