Various bugfixes and new mad scientist option

Fix One: Don't display duplicate death messages (preferring nightroles over chained kills for purposes of who actually did the kill, which matters for vengeful ghost). Closes #56.

Fix Two: Allow hunter to change their kill at night by using !kill (similar to how wolves can). Formerly, they needed to !retract first in order to change, which was nonintuitive as !retract isn't advertised anywhere.

New: Option for mad scientist to skip over dead players when determining who is adjacent to them. This is based on the number of players who have !joined the game, so you can set it so it only skips over for larger games if needed. The default is 99, which is effectively the same as the current behavior of not skipping. This allows #55 to be implemented if we decided we wanted it.