Exercise XX - caesar cipher

Implement the legendary caesar cipher:

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, 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.

write a function that takes a string to be encoded and a shift factor and then returns the encoded string:

caesar('A', 1) // simply shifts the letter by 1: returns 'B'

the cipher should retain capitalization:

caesar('Hey', 5) // returns 'Mjd;

should not shift punctuation:

caesar('Hello, World!', 5) //returns 'Mjqqt, Btwqi!'

the shift should wrap around the alphabet:

caesar('Z', 1) // returns 'A'

negative numbers should work as well:

caesar('Mjqqt, Btwqi!', -5) // returns 'Hello, World!'