Immortal invis

Immortals should have an ability to go invis both from the who list and rooms separately.

@whoinvis (disappear from the who list, but not rooms)
@whovis (appear on the who list)
@invis (invisible in rooms to mortals only)
@vis (visible to all)

Commands can be named something else, just a suggestion.

Again, I don't expect this to take priority. Just throwing it up here for reference.