Invoking other commands

[Ovyerus]

Running other commands from inside a command may come in handy for some circumstances.

Need to make a decision on how this would look syntax wise, some ideas:

• async Context#invokeOn(cmd: string)
• async Context#invokeOn<C extends Command>(cmd: C)
• async Command#invokeWith(ctx: Context)
• Just invoke instead of invokeWith or invokeOn?