Caesar"s Cipher exercise from Ironhack Web Dev Bootcamp

This is an encryption method used by Caesar to send messages to his soldiers.

Rules:

1. The user is asked to write an input. No numbers or accents are allowed (as I couldn"t figure out how to make the new numerical index conversion).
2. The user is asked to write an encryption rule (integer, X). That number will move the letters into the next letters of the alphabet X times.
3. The programm returns an array with the encryption.