Add split_every function

[Yaliang]

split_every(string) → array
Splits string on every character and returns an array.

split_every(string, length) → array
Splits string on every length characters and returns an array. length must be a positive number.

split_every(string, length, limit) → array
Splits string on every length characters and returns an array of size at most limit. The last element in the array always contains everything left in the string. limit must be a positive number.