Native support for collecting multiple arguments into a collection

[Kixunil]

It's currently possible to collect multiple arguments by using custom merge_fn, but that would require a newtype wrapper around the collection that implements ParseArg by inserting it into the collection. It might be nicer to avoid forcing everyone into implementing it by either:

• Providing a generic version of it
• Adding an option to specification and modifying codegen
• Possibly adding a shortcut option, but internally using that wrapper type.