Add option to set permission for optional arguments

[Rollczi]

e.g.
@Permission("flag.permission") @Flag("-s") boolean silent
@Permission("arg.permission") @Arg String Option<String> text

I don't know yet if this should be implemented, because it may be more readable to handle such cases yourself.

@Execute
void ban(@Context CommandSender sender, @Arg Duration time, @Flag("-s") boolean silent, @Join String reason) {
    if (sender.hasPermission("flag.permission") && silent) {
        // ...
    }
}