Add /back command

[captbunzo]

I'd like to add a /back command to FastTravel which allows a player to return to their former location. For regular players this might only be available on death.

However for staff (who often need to visit other players as a part of staff duty) it would be good for this also to work after responding to a /visit request and for admins when having used /tp to visit a player.

Perhaps other scenarios warrant consideration as well.

(moved here from nyvaria/MinecraftServer#5)