Add methods for attributes passed to liquid.

This allows plugin authors to alias this method and add new attributes to
expose to their templates. Example:

class Jekyll::Post
 alias_method :old_attributes_for_liquid, :attributes_for_liquid

  def attributes_for_liquid
   old_attributes_for_liquid.concat(%w[
     date_formatted
     updated
     updated_formatted
   ])
 end end

This would expose all the standard attributes for liquid in addition to
plugin-specific ones, which in this case are "date_formatted",
"updated", and "updated_formatted".