Multiple Requests toggling, warps and titles fix

[ + ] Made the multiple requests feature optional but enabled by default (see allow-multiple-requests) - as per #33 (untested).

[ + ] Added an option deciding as to whether or not the plugin should tell their teleport request has been displaced or expired (notify-on-expire).

[ + ] Fixed warps not deleting fully when doing /delwarp (they did get deleted but still appeared in the /warps list and could be teleported to - following restart they disappeared)

[ + ] Added a "cancelled" title to match with the teleporting title.

[ + ] Fixed titles conflicting when one is sent after the other - now the newest title takes priority.