Kata

In cryptography, a 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. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.

Your task is to write a function that takes exactly 2 arguments (string, shiftkey) and encrypts the given string. Any other character than isn't a letter should stay unchanged.

Assumption: shiftkey is integer from [-25, 25] interval.

For example:

• caesar("Abcd", 2) should return "Cdef"
  
• caesar("message", -1) should return "ldrrzfd"
  
• caesar("ZZ Top", 3) should return "CC Wrs"
  and so on ...