Given a string and integer shift implement a function which shifts each character in that string by shift amount (cesar cipher) eg. if shift is 1 then a becomes b, c becomesd, z
became a etc. Allowed input is string containing only lowercase characters from alphabet abcdefghijklmnopqrstuvwxyz.
caesar_cipher("abc", 1) # bcd
caesar_cipher("abc", 3) # def
caesar_cipher("xyz", 1) # yza