Problem 1953. Simple Caesar Cypher - shift encrypt a message given an index number

A Caesar cypher is a simple shift encryption method. Your goal is to create a function that allows a user to input a string and a shift index and outputs the shifted cypher-text as a string.

The shift index is an integer that tells the encryption method how many letters to shift each letter in the message forward. For example, a shift index of 3 will shift the letter "a" to be "d", "b" to be "e" and so on. In this problem we will just use the 26 lowercase letters in the english alphabet, and any capital letters in the message should be converted to lowercase. The ordering of letters will wrap around after "z", so that a "y" shifted by 3 will become a "b".

Any integer number (positive, negative, and of any magnitude) should be acceptable as the shift index.

Examples:

caesarShift('zebra',7) --> 'gliyh'

caesarShift('LiOn',-5) --> 'gdji'