Format numbers with separators

[bitlux]

Would you accept a PR that added an Option that formatted numbers with a separator, e.g. OptionSeparator("_") would format numbers as 1_048_576?

[schollz]

Yep

[goyal-aman]

@bitlux this change looks like a good-first-issue, could you please share the requirement in more detail, I would like to get my hands dirty on it.