var nums = [1,2,3,4,5,6,7,8,9,10];
inGroupsOf(nums, 5)
// => [[1,2,3,4,5], [6,7,8,9,10]]
inGroupsOf(nums, 4)
// => [[1,2,3,4], [5,6,7,8], [9,10]]
inGroupsOf(nums, 2)
//=> [[1,2], [3,4], [5,6], [7,8], [9,10]]