function flatMap(collection, iteratee) {
      return baseFlatten(map(collection, iteratee), 1);
    }
Creates a flattened array of values by running each element in `collection`
thru `iteratee` and flattening the mapped results. The iteratee is invoked
with three arguments: (value, index|key, collection).

@static
@memberOf _
@since 4.0.0
@category Collection
@param {Array|Object} collection The collection to iterate over.
@param {Function} [iteratee=_.identity] The function invoked per iteration.
@returns {Array} Returns the new flattened array.
@example

function duplicate(n) {
  return [n, n];
}

_.flatMap([1, 2], duplicate);
// => [1, 1, 2, 2]