format modifier for international currency symbol

Added a new modifier for the format string to use the international currency symbol instead of the local sign:

$f3->set('LANGUAGE','en-US');
$f3->format('{0,number, currency}', 12.99); // $12.99
$f3->format('{0,number, currency, int}', 12.99); // USD 12.99

$f3->set('LANGUAGE','de-DE');
$f3->format('{0,number, currency}', 12.99); // 12,99 €
$f3->format('{0,number, currency, int}', 12.99); // 12,99 EUR