[feature request] support for non-3 digit groupings

[andrewzah]

In some localizations, numbers are grouped in blocks of 4, instead of 3. It would be nice to have an option to pass this in when calling the string method.

If you'd like, I can take a stab at implementing this sometime if you give me some pointers.

[saghm]

Hi! Sorry for the late response. Someone actually wrote a much better version of this crate called num-format which supports features like this, so I expect that for most (if not all) users, it will be a better choice.