WIP: Command groups

[VoxSecundus]

This PR introduces the ability for apps using Commander to define command groups/subcommands.

Overview

The syntax for groups is similar to how one would define a command. A group is given a name, a syntax string, and optionally a summary/description. Groups are displayed at the top-level help dialog alongside ungrouped commands (grouped commands are no longer shown in the top-level help dialog). Each group has its own help dialog describing the subcommands within that group, which is shown when the user enters the group name without specifying a subcommand.

In-practise

The following block will define:

• A group named files; with:
  • A summary
  • A description
  • Two subcommands
• An ungrouped command named set-default

group :files do |g|
  g.summary = 'Perform file operations'
  g.syntax = 'files SUBCOMMAND'

  g.command :list do |c|
    c.description = 'List files in the specified directory'
    c.syntax = "files list"
    c.action ListCommand
  end

  g.command :delete do |c|
    c.description = 'Delete specified file'
    c.syntax = "files delete"
    c.action DeleteCommand
    c.slop.bool '-r', '--recursive', 'Delete a directory and all of its contents'
  end
end

command 'set-default' do |c|
  c.syntax = 'set-default'
  c.description = 'Set or view the default directory'
  c.action SetDefaultCommand
end

The default help dialog for the above would resemble the following:

  NAME:

    program_name

  DESCRIPTION:

    program_description

  COMMANDS:
	
    files       Perform file operations		
    help        Display global or [command] help documentation		
    set-default Set or view the default directory	

  GLOBAL OPTIONS:
	
    -h, --help
        Display help documentation
	
    --version
        Display version information

With the help dialog for files resembling the following:

 SYNOPSIS:

    files SUBCOMMAND

  DESCRIPTION:

    Perform file operations

  SUBCOMMANDS:

    delete Delete the specified file
    list   List files in the specified directory

Future enhancements

Currently, we only allow single-level groups; as in, a group can only contain subcommands. In the future, it may be useful to implement group nesting.