Make persistence of saved variables on player death optional

[mezzodrinker]

With the introduction of capabilities in Minecraft 1.12, saved variables need to be migrated from the dead player to the respawned one. This makes it easy to “drop” the variables when the player dies. Default behaviour should stay the same as before so as to not “surprise” players, but for people who would like to have this sort of feature, a configuration option could be provided.