[Feature Request] Ability to specify default value(s) for slice flags

[andoks]

I would love to have the ability to specify default value(s) for slice flags in the "new" (struct tag) way, as AFAICS there is not way to specify default values for slice flags that are part of subcommands (with top-level struct-args one can use the old-style way of setting the slice's value before parsing the flags)