Introduce --help support for all subcommands (recursively)

[jdegoes]

Currently, --help is only available for the top-level command. This is inconvenient, because we have full documentation for all subcommands. Yet, at the same time, we cannot display all documentation on a single page. We should wait until the user requests help for a subcommand to print the help.

Thus, our subcommand help can work a bit like git:

git fetch --help

Now, in order to make this work, we need to take care of a few details:

1. First, the way subcommand help is printed out will be somewhat different from the way the top-level command is printed out. We should look to git and other complicated CLI tools for inspiration.
2. Second, we need to ensure we can print out help even if a parent command does not have all of its required options and arguments. If we were to do a simple process of augmenting the child commands with --help, then we would only succeed in parsing them if the parents succeeded too. But for help, we really want to ignore required arguments/options for parents, and give the user help anyway. We need to make sure this is possible.
3. Third, we need to ensure the way it is rendered looks good for commands with subcommands, both the top-level help, as well as the subcommand help, and that it scales for subcommands of subcommands of subcommands (etc.).

[hollinwilkins]

Happy to take a look at this one