Added the AvatarStateOnFinalBlow and Randomly Select a Player's Element When They Join

When a player with access to the Avatar State is going to receive a
killing blow, they will not die and will access the Avatar State. This
is able to be enabled and disabled in the config.

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A player is randomly assigned to be either a chiblocker or a bender.
This can be disabled, the percentages are customizable, and the default
element if they don't receive the others is also customizable.