Possibility to show value in opposite to default currency

My default currency is PLN (in config.php)
```
// 1 PLN = X USD
@currency(1, 'USD', null) // -> 0.32
```

Now with `$inverse` parameter:
```
// 1 USD = X PLN
@currency(1, 'USD', null, true) // -> 3.09
```

I don't know how to name this parameter in English, so you can change it to a more appropriate.