0.9.8 - Api improvements

Two main changes with this update:

• OnHelpInvoked now provides a default implementation, as feedbackwas that it's a bit confusing that --help does nothing without implementing the virtual method. Now we implement the virtual method to change the implementation
• Enforce at least one name being provided by Option/OptionParameter attributes, whereas it was possible to provide neither, before.

The Api is getting close to complete now, and no more significant changes to existing functionality are proposed.