Added a way to control the teleportation when the spectator mode is changed.

There is now two new options:
 - one to disable the teleportation if there isn't any main lobby set;
 - the other one to use the spectator's world' spawn point instead of the /spawn command (useful if there isn't any /spawn command in the server).

Also, the /spawn command is executed only if available, to avoid the « Unknow command » message.