Option to use number for user and/or group names

[breezerider]

• add NumericUid and NumericGid to Tar struct
• writeFileToArchive: set Uname and Gname to empty string if NumericUid and NumericGid are true, respectively.

[mholt]

Thanks! Is there a reason one would be true but not the other?

[breezerider]

No real use case on my side, just left it like that for flexibility. Primary use case is to prevent aliasing and this means both flags are set simultaneously. Do you think it makes sense to join them?

[mholt]

I think we should probably join them unless there's a valid use case for having them separated. We're still in alpha so we could change this later if there's a valid use case brought forth.

[mholt]

Thanks! This is good. I'm just changing UidGid to UIDGID, as dumb as that looks, because the convention I'm used to capitalizes initialisms 🙃

I appreciate the change!!