Remove final modifier from NamedOptionDef

[rensink]

I want to create an option whose usage message is derived dynamically from other info in the program. That won't work for the usage-parameter in the @Option as that needs to be known at compile time. The easiest way to achieve this would be to subclass NamedOptionDef and overwrite OptionDef#usage, but that is blocked by the fact that NamedOptionDef is final.

I propose to remove the final modifier.