Allow defining named teleport points

[Christian Illy]

As per mail request:

Specifically, I would love to be able to assign a name to a set of
coordinates, which would be substituted at command execution. E.g.,

alias CoolLocation 250 -1 300
tele PlayerName CoolLocation

Ideally, it would be persistent (saved to a text file or sqlite db), but
still very useful and acceptable if the aliases only stay in memory
until next reboot. (I know it could get complicated to parse the file,
make sure the alias doesn't already exist, delete old aliases, etc.)