Add support for command group captions

[DannyBen]

Add the ability to create a usage text like this:

ftp - Sample application with command grouping

Usage:
  ftp [command] [options]
  ftp [command] --help | -h
  ftp --version

File Commands:
  download   Download a file
  upload     Upload a file

Login Commands:
  login      Write login credentials to the config file
  logout     Delete login credentials to the config file

By adding group caption in the first command in each group, like this:

name: ftp
help: Sample application with command grouping
version: 0.1.0

commands:
- name: download
  help: Download a file
  group: File

  args:
  - name: file
    required: true
    help: File to download

- name: upload
  help: Upload a file

  args:
  - name: file
    required: true
    help: File to upload

- name: login
  help: Write login credentials to the config file
  group: Login

- name: logout
  help: Delete login credentials to the config file

In addition, rename subcommands to commands in many places, ,to be consistent. We now use subcommands to state that this is a nested command.