Add quantity to random spawns

Summary

Add a new option, such as "Amount" or "GroupNumber" for random spawning system to set a number or range that how many mobs will be spawned as a group at a spawn point. For example, if set "GroupNumber: 2-5", there will be 2-5 mobs being spawned at each point.

Usage

To randomly spawn groups of mobs in a single configuration.

Value

Makes the random spawn system more like the vanilla mob spawning. Most vanilla mobs, such as zombies, pigs, are usually spawned as a group containing 1-4 mobs together.