Support specifying format via I18n

[TastyPi]

What's changing?

Allow the default format for a language be configured via I18n.

Why?

Fixes #233 (which was closed with no solution).

This is blocking us from launching our product in France.

[TastyPi]

It uses the key number.currency.format.format, which I don't love, suggestions welcome.

[ocarreterom]

Maybe this code could be move to determine_format_from_formatting_rules since :symbol_position, :symbol_before_without_space and :symbol_after_without_space are deprecated options.

Something like:

def determine_format_from_formatting_rules(rules)
  begin
    I18n.t :format, scope: "number.currency.format", raise: true
  rescue I18n::MissingTranslationData
    return currency.format if currency.format && !rules.has_key?(:symbol_position)
    symbol_position = symbol_position_from(rules)
    if symbol_position == :before
      rules.fetch(:symbol_before_without_space, true) ? '%u%n' : '%u %n'
    else
      rules[:symbol_after_without_space] ? '%n%u' : '%n %u'
    end
  end
end