Implement currency formatting

- [x] Implementation
- [x] Tests
- [x] Doc

Fix #238 
Instead of...

```js
fn = { code: currencyCodeProperties, name: currencyNameProperties };
properties = ( fn[ options.style ] || currencySymbolPattern )( currency, cldr, options );
```

An alternative code is...

```js
if ( options.style === "code" ) {
    properties = currencyCodeProperties( currency, cldr );
} else if ( options.style === "name" ) {
    properties = currencyNameProperties( currency, cldr );
} else {
    properties = currencySymbolPattern( currency, cldr, options );
}
```