Option to display None default?

[nyoma-diamond]

If it does not conflict with how argparse works, I think it would be useful to have an option that allows explicitly displaying when the developer has specified that an argument defaults to None internally if not provided. This is useful for arguments where the default case shares some, but not all, functionality with another case that may appear the same to the use but actually isn't. E.g., in my project None and 0 have very similar, but non-identical behavior, as None ensures the value is ignored, while 0 specifies to consider it, but treat it as 0, which doesn't do exactly the same thing.