command option to print hierarchy

(Optionally-enabled) (manage) command/option or other tool to print command hierarchy, like `tree`, (with descriptions and/or argument signatures).

For example:

$ manage server --help-tree
manage server manage servers
|– user manage users via ansible
| |– add add user logins
| |– delete mark logins for deletion
…

Above, the `--help-tree` option operates like `--help`, immediately printing a description of the hierarchy and exiting.

This could be offered by the `manage` command, but *not* enabled by default for other commands that make use of `argcmdr`; rather, for these, it should be easy to add or enable.