Set default values for site variables

good for gem-based first run

_layouts/home.html

@@ -83,7 +83,7 @@ layout: page
       {% if post.pin %}
       <div class="pin">
         <i class="fas fa-thumbtack fa-fw"></i>
-        <span>{{ site.data.label.pin_prompt }}</span>
+        <span>{{ site.data.label.pin_prompt | default: 'Pinned' }}</span>
       </div>
       {% endif %}