HotPotato: show the pass history

[brenns10]

Issue created on behalf of @MatthewBentley.

Would be useful if we could see the pass history for the hot potato. There could be a lot of detail here because I actually keep timestamps for each pass, but I think just the following format:

user1 -> user2 -> user3 -> user4

would be fine.

Question / feedback items:

1. Should this be a separate command, or should it be part of slacksoc who has the potato?
2. Should there be messages in #random every time someone passes the potato?