Add support for captioning images in feature row helper

Use `image_caption` YAML front matter to assign a caption to the feature image, Markdown is allowed.

Close #1440

_includes/feature_row

@@ -25,6 +25,9 @@
                 "{{ f.image_path | relative_url }}"
               {% endif %}
             alt="{% if f.alt %}{{ f.alt }}{% endif %}">
+            {% if f.image_caption %}
+              <span class="archive__item-caption">{{ f.image_caption | markdownify | remove: "<p>" | remove: "</p>" }}</span>
+            {% endif %}
           </div>
         {% endif %}