Support populating custom field types (not just arrays and Collections) from multiple options/parameters

[remkop]

e.g. <command> -x 123 "name" HOURS

@Option(names = "-x", type = {Integer.class, String.class, TimeUnit.class}, arity = "3")
MyBean mybean;   // TBD arity must match the number of classes in the type attribute?

where

class MyBean { // all that is required is a public constructor matching the specified types
    public MyBean(int number, String string, TimeUnit timeUnit) {
        this.number = number;
        this.string= string;
        this.timeUnit= timeUnit;
    }
    ....

---

Of course, -x 123,name,HOURS could also be converted by a custom ITypeConverter

[remkop]

Somewhat related to #358.

[remkop]

Picocli 4.0.0-alpha-1 has been released which includes support for repeating composite groups,. This should address this use case.
See https://picocli.info/#_argument_groups for details.