Suggestion: add "portable" option to pass to tar.Pack

[murgatroid99]

I have recently encountering some issues where I package and distribute tarballs as a non-root user, then someone tries to extract it as root and it fails because the ownership data is preserved and it is trying to assign file ownership to a nonexistent user. tar.Pack's portable option looks like it would solve that because it omits uid and gid metadata, so I suggest that that option be made available in this library's pack function.

[murgatroid99]

In addition, for similar reasons, I think it would be useful to expose the tar.Unpack's preserveOwner option. I would like to note that the default for that option depends on external factors (specifically the current process's user ID), so in order to avoid changing behavior in the default case, this library should pass undefined through unchanged instead of choosing a default of true or false.