Fix #830 - Recognize Option, Value and Usage attributes on private members

[Saalvage]

Potential use cases:

• Requiring additional parsing which requires type or multiplicity changes and wanting to avoid exposing those init/set-only properties unnecessarily, as usage of the Options class from a programming interface should not rely on the parsing and instead assign the parsed properties directly.
• The ability to encapsulate functionality in the Options class without having to expose the properties directly.
• Not wanting to expose the Usage functionality to the programming interface.

Additional arguments:

• It's already possible to use private init/set on a public property, so this seems more like an oversight (or the other way around!)
• I don't think there are any good reasons to uphold this restriction.

[ericnewton76]

The point of the Options class was only to hold the parser results. Its a single-use design, that implies you have the ability to make public all options so the parser can apply parsed items from the commandline.

I'm a little dubious on making the change but since it doesnt actually hurt anything it'll probably go through

And BTW, thank you for also submitting the tests.