Provide option to format `0`

[ckruse]

Hey there,

the current implementation hard-codes 0 to "0":

 } else if num.is_zero() {
            "0"

This means, we can't format zero values, e.g.

    let mut fmt = numfmt::Formatter::new()
        .separator('.')?
        .precision(numfmt::Precision::Decimals(2))
        .suffix(" €")?;

    let num = fmt.fmt2(*value);

Now num is just "0" instead of "0,00 €".

Can you provide an option to also format 0?