Allow to sell item for: money, extendedCost or money+extendedCost.

The new method (money+extende) can be used by using negative values on npc_vendor.ExtendedCost.
This one goes dedicated to ZxBiohazardZx.
Tnx also to VladimirMangos.

--HG--
branch : trunk

src/server/game/Chat/Commands/Level2.cpp

@@ -1051,7 +1051,7 @@ bool ChatHandler::HandleNpcAddVendorItemCommand(const char* args)
         incrtime = atol(fincrtime);
 
     char* fextendedcost = strtok(NULL, " ");                //add ExtendedCost, default: 0
-    uint32 extendedcost = fextendedcost ? atol(fextendedcost) : 0;
+    int32 extendedcost = fextendedcost ? atol(fextendedcost) : 0;
 
     Creature* vendor = getSelectedCreature();