Clean up prune.sh

* Exit with an error if level-name not defined in server.properties
* Allow block IDs without trailing comments in whitelist file
* Also let any whitespace character follow the block id
* Make blockfile() function use its argument
Add pregen.sh example script

This script pregenerates a square of land, much like mcexplore
(or prune.sh) does, but it accomplishes it by teleporting a player
around rather than repeatedly moving the spawn point and bouncing
the server.