Set default values for site variables

good for gem-based first run

_layouts/category.html

@@ -14,11 +14,13 @@ layout: page
   </h1>
 
   <ul class="post-content pl-0">
+    {% assign post_long_df = site.data.date_format.post.long | default: '%b %e, %Y' %}
+
     {% for post in page.posts %}
     <li class="d-flex justify-content-between pl-md-3 pr-md-3">
       <a href="{{ post.url | relative_url }}">{{ post.title }}</a>
       <span class="dash flex-grow-1"></span>
-      <span class="text-muted small">{{ post.date | date: site.data.date_format.post.long }}</span>
+      <span class="text-muted small">{{ post.date | date: post_long_df }}</span>
     </li>
     {% endfor %}
   </ul>