Add image_destination to config['keep_files'] in #render (@parkr)

Resolves #3 and #4

picture_tag.rb

@@ -56,6 +56,9 @@ def render(context)
       image_source = settings['source_path'] || '.'
       image_dest = settings['output_path'] || File.join(image_source, 'generated')
 
+      # Prevent Jekyll from erasing our copied files
+      site.config['keep_files'] << image_dest unless site.config['keep_files'].include?(image_dest)
+
       # Deep copy preset to sources for single instance manipulation
       sources = Marshal.load(Marshal.dump(settings['presets'][@preset]))
 
@@ -220,12 +223,6 @@ def generate_image(source, site_source, site_dest, image_source, image_dest)
       gen_jekyll_path
     end
   end
-
-  # Patch to prevent Jekyll from erasing our copied files (there's probably a less ugly way to do this)
-  class Configuration < Hash
-    picture =  Jekyll.configuration({})['picture']
-    DEFAULTS['keep_files'] = DEFAULTS['keep_files'].push(picture['output_path'] || 'generated') unless picture.nil?
-  end
 end
 
 Liquid::Template.register_tag('picture', Jekyll::Picture)