Support case classes in constructor arguments to group options for re-use.

[nh13]

It would like case classes to be a parameter in a constructor for a command line, but each member of that case class be exposed on the command line. This would allow a case class to be re-used across multiple classes' constructors:

case class Pirate(@arg(doc="arrrghh") val argh: String, (@arg(doc="matey") val matey: String)
class ConstructPirateShip(val pirate: Pirate)
class ConstructFrigate(val pirate: Pirate)

would show help for --argh and --matey for both classes above.