feature request: collect multiple args into other containers than Vec

[nagisa]

Custom parsers appear to only go from a single value to another kind of type, but there’s no way to do e.g.

#[structopt(multiple=true, takes_value=true, ...?)]
something: std::collections::HashSet<...>,

that I can see. It becomes necessary to convert from Vec to HashSet or similar containers after arguments are parsed.

[TeXitoi]

Duplicates of #79

[TeXitoi]

I agree that it would be a great feature. That's one of the oldest proposition, but we don't have any viable proposition, neither any motivated contributor yet.