Add support for gallery image captions in Magnific Popup overlays

- Apply optional `title` attribute to image anchors that trigger Magnific Popup overlays
- Close #334

_includes/gallery

@@ -25,6 +25,7 @@
         {% else %}
           "{{ img.url | prepend: "/images/" | prepend: base_path }}"
         {% endif %}
+        {% if img.title %}title="{{ img.title }}"{% endif %}
       >
         <img src=
           {% if img.image_path contains "http" %}