Doc: Add currency formatting

- Add `.currencyFormatter( currency [, options] )`;
- Add `.formatCurrency( value, currency [, options] )`;

Ref #238
Doc: Change plural example

Amends "Plural: Create generator method".
Amends 3e1626e386e13269bac86d503d6dd3c3e9656f25
Ref #335
Ref #336
Currency: Implement currency formatting (accounting style)

Ref #238