Deal with lists of values

[tertsdiepraam]

There are a few options in coreutils which accept lists of values, e.g.

• cut --fields
• chroot --groups

Sometimes these are additive, sometimes they override and sometimes they can only be passed once.

We need to figure out a way to deal with these case. The overriding case is easy (just parse into a vec and override the vec like any other value). Additive and only once are harder cases.

[tertsdiepraam]

Well, can't find any additive cases and the set-only-once is the same case as https://github.com/tertsdiepraam/uutils-args/issues/11