Quantity.toString receiving a DecimalFormat

Quantity could have the following methods:

```scala
import java.text.DecimalFormat

def toString(decimalFormat: DecimalFormat, uom: UnitOfMeasure[A]): String =
  s"${decimalFormat.format(to(uom))} ${uom.symbol}"
def toString(decimalFormat: DecimalFormat): String =
  toString(decimalFormat, unit)
```

DecimalFormat is locale aware so on JS this might not work out of the box.