def value_for(item, option)
      attribute = @options[option]
      if attribute.is_a? Proc
        attribute.call(item)
      else
        eval("item.#{attribute}")
      end
    end
def value_for(item, option)
      attribute = @options[option]
      if attribute.is_a? Proc
        attribute.call(item)
      else
        eval("item.#{attribute}")
      end
    end
Returns value of item's attribute