Define converter on Command.

Prepares for future dev on validating unparsed arguments on a command
specific basis.
0.6.2
Support recursive toObject().
0.6.3
Support item callback in toObject().
Define options() and commands() mutators.
Assert on class conversion.

Such that options() and commands() will coerce plain objects with a name
property to the appropriate class: Flag, Option or Command.
0.6.4