This feature can be implemented in multiple PRs but it's likely that the required changes for styling one of the elements will make easy to style the others. In that case, a single PR will probably be enough.

• metavars (including choices)
• default values
• environmnent variables.

Originally proposed in #95. Closed that one because it proposes a specific regex-based approach that I think it's not suitable for Cloup. This issue is for discussing different approaches, just in case anyone would like to.

Questions:

• is the needed work worth it or the current theming is good enough?
• given that, in theory, Click will have a new formatting system soon or later, is this the right time to implement this feature?
• wouldn't be better to change approach completely and show a web page when the user type --help and there's a browser available (similarly to git)? Command line will never be more readable :)