def encoding_option(opts)
      encoding_desc = 'Specify the default encoding for input files.'
      opts.on('-E', '--default-encoding ENCODING', encoding_desc) do |encoding|
        Encoding.default_external = encoding
      end
    end
def encoding_option(opts)
      encoding_desc = 'Specify the default encoding for input files.'
      opts.on('-E', '--default-encoding ENCODING', encoding_desc) do |encoding|
        Encoding.default_external = encoding
      end
    end
Set an option for specifying `Encoding.default_external`.

 @param opts [OptionParser]