Add possibility to create subcommands

[mgrosperrin]

The goal is to be able to create subcommand for a givent command.

To call the subcommand, the arguments should be <command> <subcommand> [<subcommand options>].
If an option of the "main" command is specified before the name of the sub-command, the name of the sub-command will be treated like an arguments (retrievable in the Arguments property).