Enhanced top-level command support (`say`, `exec`, Command Extensions...)

[apocalyptech]

It'd be nice if say and exec were handled "properly," in that they could be toggled on+off like all the usual set commands. Relatedly, it'd be nice to have the same support for extra commands added via Command Extensions, though that might be better off as a separate issue entirely, since that would also require support for the user/mod being able to define what new commands have been defined.