Ability to disable commands completely

[paulodiovani]

Is your feature request related to a problem? Please describe.
Default commands from https://github.com/ianyh/Amethyst/blob/development/Amethyst/default.amethyst are always present, unless overriden with a new keybinding, when using a config file. But some of them might not be wanted by some users, and we should have a way to disable them completely to avoid triggering them unintentionally.

Describe the solution you'd like
Set a command to null on the amethyst.yml.

E.g.

select-wide-layout: null

Note: Amethyst should NOT use the default keybinding.

Describe alternatives you've considered
Currently, the alternative is to set a keybinding unlikely to be triggered.

Additional context
My dot file for reference: https://github.com/paulodiovani/dot-files/blob/feature/macos-v2/home/user/.config/amethyst/amethyst.yml