Unavailable commands

[WesCossick]

Commands, since their specifications are defined dynamically, should allow for an unavailable: boolean; key. If true, then that command should appear grayed out in the help screen and any attempt to use it should fail.

We could also allow for defining a custom explanation of why the command is unavailable, so the end user knows why they can't use it. It could appear in the help screen and definitely in the error message if they try to use it.