Accounting friendly negative number formatting. By configuring the `negative_format`, Rail's `number_to_currency` helper method will display negative numbers with parenthesis.

en.yml

  # find this file in config/locales/en.yml
  # http://guides.rubyonrails.org/i18n.html
  # https://github.com/svenfuchs/rails-i18n/blob/master/rails/locale/en.yml
  number:
    currency:
      format:
        delimiter: ","
        format: "%u%n"
        negative_format: "(%u%n)"  # <-- wrap negative numbers in parenthesis
        precision: 2
        separator: .
        significant: false
        strip_insignificant_zeros: false
        unit: $