Opt args that take default value when skipped

[Atreyagaurav]

Fixes #325

• a global #[default] attribute to enable the feature on the test function and all arguments are considered optional and use default
• if some argument have also the #[case(opt)] we wrap it with Some() if present and None otherwise
• Raise error if some arguments are optional, but the global default is not present.

[Atreyagaurav]

Please refer to the integration tests for how to use it. And suggest any improvements I should do on this implementation.

I need more time to think about implementing the case(opt) for individual args.

Aside: It'd be nice if we could give values from case() with key = value, that way we could omit values irrespective of their locations, but that seems more trouble than worth it at the moment.

[la10736]

Thanks for your contribution. I'll review you PR ASAM... but I'm quite busy in these days.

[Atreyagaurav]

No worries, I'm still working on the last two, so you don't need to do a thorough review. Just letting me know if I'm on the right track is enough for now. Whenever you can is fine.

[la10736]

Sorry for my late answer. I're going in the right direction!