Allow markdown in a list's subtitle field.

layouts/_default/list.html

@@ -4,7 +4,7 @@
     <main class="posts">
         <h1>{{ .Title }}</h1>
         {{ if .Params.subtitle }}
-        <strong>{{ .Params.subtitle }}</strong>
+        <strong>{{ .Params.subtitle | markdownify }}</strong>
         {{ end }}
 
         {{ if .Content }}