Support platform-dependent options

[tdelacour]

It would be convenient if there was built-in functionality for a programmer to specify a platform dependency on a given option.

For example, something like: type Options struct { WindowsOption bool 'long:"windowsOption" description:"only shows up on windows builds" platforms:"windows"' }

[jessevdk]

I don't want to add more tags for minor (or limited use case) features. I'd rather consider adding parser.AddOption so you can programatically add such options at runtime on a conditional basis.

[tdelacour]

That strikes me as a good alternative!