Feature/subcommand

[muunitnocQ]

Closes #17 Adds a new marker annotation called QualifiedSubCommand:

@QualifiedSubCommand(aliases = {"commandalias"})
class MySubCommand extends SubCommand {
   //....
}

This registers the subcommand as it would normally, but also registers it as a base command. The annotation optionally takes an aliases parameter.

Disclaimer: I have no means of testing this. I don't see why it wouldn't work, but yeah.