User definition sprockets function

sprockets-extend.rb

module Sprockets::SassProcessor::Functions
  def cdn_path(path, options={})
    prefix = if (asset_host = Rails.application.config.action_controller.asset_host)
               asset_host.scan(%r(/[^/]+\Z)).first
             else
               ""
             end
    prefixed = prefix + asset_path(path, options).to_s
    Sass::Script::String.new(prefixed)
  end
end