Fix #3: add categories support

layouts/_default/single.html

@@ -9,6 +9,16 @@
             {{ if .Params.date }}
             <div class="article-date">{{ .Params.date.Format "2006-01-02" }}</div>
             {{ end }}
+            {{ if .Params.categories }}
+            <ul class="article-categories">
+                {{ range .Params.categories }}
+                    <li><a href="/categories/{{ . }}">{{ . }}</a></li>
+                {{ end }}
+            </ul>
+            {{ end }}
+            {{ if and (.Params.categories) (.Params.tags) }} 
+                |
+            {{ end }}
             {{ if .Params.tags }}
             <ul class="article-tags">
                 {{ range .Params.tags }}