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.