support providing a metadata file

layouts/partials/slideshow.html

@@ -4,9 +4,14 @@
   {{- range $i, $img := $imgs }}
   <div class="snap-image">
     <div class="numbertext">{{ add $i 1 }} / {{ len $imgs }}</div>
-    <img src="{{ $img }}" onclick="openLightbox({{ $galno }});openLightboxItem({{ $galno }}, {{ add $i 1 }});">
+    <img
+      src="{{ relURL $img.src }}"
+      {{ range $attr, $value := $img.html -}}
+        {{ safeHTMLAttr $attr }}={{ $value }}
+      {{ end -}}
+      onclick="openLightbox({{ $galno }});openLightboxItem({{ $galno }}, {{ add $i 1 }});" />
   </div>
-  {{- end -}}
+  {{- end }}
 
   <a class="snap-prev" onclick="moveSlideshowItem({{ $galno }}, -1)"><span>&#10094;</span></a>
   <a class="snap-next" onclick="moveSlideshowItem({{ $galno }}, 1)"><span>&#10095;</span></a>