Improve help text formatting

This is a first iteration towards a help text output that works better for parsers with nested commands. I think what should be improved is the fact that now the braces around the commands and the rest of the mandatory fields are rather hard to parse (it's correct, but not easy to see). Also, I thought about inserting line breaks between the help and version flags, but it makes things more difficult because we probably have to indent the commands then so it's clear which one's which. The same goes for parsers with deeper nested subcommands. There are braces around the different subcommands now (which helps in distinguishing them), but if the subcommands themselves require several configurations, things get messy again.