Implement the spellbook of chain lightning

Prior to this commit, there was no good way to deal with swarms of
weak, good-AC enemies using magic; trying to play a wizard as a
pure spellcaster would make bees and ants very difficult to deal
with (because they would usually dodge force bolts).

This commit adds a new spell designed to be very good against
swarms of weak enemies: "chain lightning", which does 2d6 lightning
damage to every monster around you. It has an initially short range,
but can chain from monster to monster to cover potentially large
distances (as long as none of the monsters en route are shock
resistant or tame/peaceful; the spell can't chain past shock
resistant monsters and avoids peacefuls). Monsters within one
space of the visible lightning bolts are affected. Unlike other
lightning effects, this one does only 2d6 damage, not enough to
blind affected monsters.

This commit breaks existing save and bones files (thus the
EDITLEVEL increase).

include/patchlevel.h

@@ -17,7 +17,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 91
+#define EDITLEVEL 92
 
 /*
  * Development status possibilities.