feat: enable optional custom date format

Adds a .Site.Params.dateFormat setting to let users specify a Hugo-style
date format string for content. Previously, the date format was
hardcoded to "02 Jan, 2006", presumably to match the original BearBlog
style. This format gets used by default if the new "dateFormat" setting
is absent from the site config.

layouts/_default/list.html

@@ -12,7 +12,7 @@
       <span>
         <i>
           <time datetime='{{ .Date.Format "2006-01-02" }}' pubdate>
-            {{ .Date.Format "02 Jan, 2006" }}
+            {{ .Date.Format (default "02 Jan, 2006" .Site.Params.dateFormat) }}
           </time>
         </i>
       </span>