One of the simplest and most widely known ciphers is a
Caesar cipher, also known as a shift cipher. In a shift cipher the meanings of the letters are shifted by some set amount.
A common modern use is the ROT13 cipher, where the values of the letters are shifted by 13 places. Thus 'A' ↔ 'N', 'B' ↔ 'O' and so on.
Write a function which takes a ROT13 encoded string as input and returns a decoded string.
All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on.
