CLI: Add per-command --help

[ee7]

The --help message is currently in the format:

$ configlet --help
configlet 4.0.0-beta.5
The official tool for managing Exercism language track repositories.

Usage:
  configlet [global-options] <command> [command-options]

Commands:
  completion  Output a completion script for a given shell
  fmt         Format the exercise '.meta/config.json' files
  generate    Generate Concept Exercise 'introduction.md' files from 'introduction.md.tpl' files
  info        Print some information about the track
  lint        Check the track configuration for correctness
  sync        Check or update Practice Exercise docs, metadata, and tests from 'problem-specifications'.
              Check or populate missing 'files' values for Concept/Practice Exercises from the track 'config.json'.
  uuid        Output new (version 4) UUIDs, suitable for the value of a 'uuid' key

Options for completion:
  -s, --shell <shell>          Choose the shell type (required)
                               Allowed values: b[ash], f[ish], z[sh]

Options for fmt:
  -e, --exercise <slug>        Only operate on this exercise
  -u, --update                 Prompt to write formatted files
  -y, --yes                    Auto-confirm the prompt from --update

Options for info:
  -o, --offline                Do not update the cached 'problem-specifications' data

Options for sync:
  -e, --exercise <slug>        Only operate on this exercise
  -o, --offline                Do not update the cached 'problem-specifications' data
  -u, --update                 Prompt to update the unsynced track data
  -y, --yes                    Auto-confirm prompts from --update for updating docs, filepaths, and metadata
      --docs                   Sync Practice Exercise '.docs/introduction.md' and '.docs/instructions.md' files
      --filepaths              Populate empty 'files' values in Concept/Practice exercise '.meta/config.json' files
      --metadata               Sync Practice Exercise '.meta/config.json' metadata values
      --tests [mode]           Sync Practice Exercise '.meta/tests.toml' files.
                               The mode value specifies how missing tests are handled when using --update.
                               Allowed values: c[hoose], i[nclude], e[xclude] (default: choose)

Options for uuid:
  -n, --num <int>              Number of UUIDs to output

Global options:
  -h, --help                   Show this help message and exit
      --version                Show this tool's version information and exit
  -t, --track-dir <dir>        Specify a track directory to use instead of the current directory
  -v, --verbosity <verbosity>  The verbosity of output.
                               Allowed values: q[uiet], n[ormal], d[etailed] (default: normal)

It would be nice to support configlet <command> --help for printing command-specific help, which makes it easier to add more detailed explanation and examples.

Previously discussed: #96 (comment).