Add API for fishing results

The problem with that is that the method to get the ItemStack from the fishable can return literally any valid ItemStack, and can have multiple different items yet be valid (e.g., if someone wanted to let you fish up ex nihilo's broken ore items). I think an interface with a method that returns a boolean for whether a given fishable should be kept or not would let people actually inspect the item (i.e., Array.filter in javascript).