Add more cowbell. Jsonize(tm) monster drops in order to do it.

Fixes https://github.com/CleverRaven/Cataclysm-DDA/issues/4420

data/json/monster_drops.json more or less follows the format in data/json/item_groups.json. No internal changes past init/load

Side note; monster drops is rather convoluted, taking cowbell as an example; to drop a *single* bell half the time one must:

  *  data/json/monsters.json : edit item_chance to *negative* 50
  *  data/item_groups.json : add "cow" item group
  *  data/monster_drops.json : add "cow" mongroup entry, which is basically a grouping of groups.

I have a few ideas for alternative means of specifying drops and may submit in another PR (whereupon clanking cowbell for extended periods in game has slight morale boost effect)