[NEW] Group top level commands by registry

[bhou]

This will improve the situation mentioned in #138

Tested the original idea of grouping top level commands by package, turns out the output message are grouped into too many fragments, which reduces the readability.

This PR only group the top-level commands by registry and it can be toggled by a new configuration item group_help_by_registry with default value true

Here is an example help:

Criteo Dev Toolkit - A command launcher 🚀 made with <3

Happy Coding!

Example:
  cdt --help

Usage:
  cdt [flags]
  cdt [command]

Commands from 'dropin' registry
  migration                                   Collection of tools for migration
  moab-report                                 Generate MOAB reports

Commands from 'default' registry
  code                                        help for the command code
  devenv                                      Manage development environment
  doc                                         generate documentation from sources
  hotfix                                      Create, review, and build hotfix from your terminal
  moab                                        Generic MOAB commands
  mozart                                      Help for the command mozart

Additional Commands:
  completion                                  Generate completion script
  config                                      Manage configurations
  help                                        Help about any command
  login                                       Login to use services
  package                                     Manage command launcher packages
  remote                                      Manage command launcher remotes
  rename                                      Rename installed command
  update                                      Update cdt, or its commands
  version                                     Print the version number of CDT command

Flags:
  -h, --help   help for cdt

Use "cdt [command] --help" for more information about a command.