Helper library for creation of CommandCallables

[sk89q]

Currently, one can make a command by extending CommandCallable. However, we probably want to provide a helper library to go along with it to make the creation of commands easier.

[matthewprenger]

Any plans to use an annotation system similar to the Intake one?

[sk89q]

Yes, at some point when I get time.

[Aaron1011]

I'd be interested in taking this on, unless @sk89q is planning to.

[sk89q]

Someone in in the process in making some changes to the command code at the moment.

[Zidane]

Covered in zml's Commands Overhaul PR.

#561