Add registers.site to context when rendering template

Fixes #101.

features/test-site/_includes/responsive-image.html

@@ -1 +1,3 @@
+{% assign largest = resized | sort: 'width' | last %}
+
 <img alt="{{ alt }}" src="/{{ original.path }}" title="{{ title }}" srcset="{% for i in resized %}/{{ i.path }} {{ i.width }}w,{% endfor %}/{{ original.path }} {{ original.width }}w">

lib/jekyll-responsive-image/renderer.rb

@@ -25,7 +25,11 @@ module Jekyll
           partial = File.read(image_template)
           template = Liquid::Template.parse(partial)
 
-          result = template.render!(@attributes.merge(@site.site_payload))
+          info = {
+            registers: { site: @site }
+          }
+
+          result = template.render!(@attributes.merge(@site.site_payload), info)
 
           RenderCache.set(cache_key, result)
         end