Use the Hugo "figure" shortcode instead of standard md images

Using the "figure" shortcode allows us to support class attributes and
captions for exported images, which are not available in
blackfriday-flavoured markdown.

Begins to address #3 and
#4.

ox-hugo.el

@@ -185,18 +185,21 @@ and rewrite link paths to make blogging more seamless."
      ((org-export-inline-image-p link org-html-inline-image-rules)
       (message "org-hugo-link processing an image %s" contents)
 
-      (let ((path (org-hugo--attachment-rewrite
+      (let* ((path (org-hugo--attachment-rewrite
                    (let ((raw-path (org-element-property :path link)))
                      (if (not (file-name-absolute-p raw-path)) raw-path
                        (expand-file-name raw-path)))
                    info))
             (caption (org-export-data
                       (org-export-get-caption
                        (org-export-get-parent-element link))
-                      info)))
-	(format "![img](%s)"
-		(if (not (org-string-nw-p caption)) path
-                  (format "%s \"%s\"" path caption)))))
+                      info))
+            (parent (org-export-get-parent link))
+            (attr (org-export-read-attribute :attr_html parent))
+            (class (plist-get attr :class)))
+	(format "{{<figure src=\"%s\" caption=\"%s\" class=\"%s\">}}"
+                path caption class
+		)))
      ((string= type "coderef")
       (let ((ref (org-element-property :path link)))
 	(format (org-export-get-coderef-format ref contents)