Caesar cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.
caesarCipher(str, num) should shift every letter in a given str by a num (positions down the alphabet).
Example:
caesarCipher('Zoo Keeper', 2) // 'Bqq Mggrgt'