Generate a single man page with all subcommands

[aparcar]

Hi, is it possible to "flatten" all subcommands into a single man page? I think it's very helpful to use help <command>, but for the man page I'd rather have a single long page instead of multiple.

[epage]

While I don't know if we support Command::flatten_help yet for man generation, that could be implemented and you could enable it only when generating man pages.