Add a function to group an array into fixed length sub-arrays.

[euoia]

For example:

> var arr = [1,2,3,4,5];
undefined
> _.grouped(arr, 3);
[ [ 1, 2, 3 ],
  [ 4, 5 ] ]

[michaelficarra]

Duplicate of #696.

[euoia]

Thanks for that. I learned that I can mix it in if I want. But I'm still not really clear why the original PR wasn't accepted - this seems like useful functionality that exists in other languages.