code: def export_to_file(filename, options={})
        content_string = export_to_string(default_options(filename).merge(:filename => filename))
        File.open(filename, "w") { |f| f.write(content_string) }
        nil
      end
docs: Export this map to the given filename in the appropriate format.
  
   @param filename [String] The file path to export to
   @param options [Hash] Options for exporting
   @option options [Symbol] format The format to export to, such as :tmx or :json
   @return [void]