caesars ciphera FreeCodeCamp challenge
function rot13(str) { // LBH QVQ VG!
    var cipher = str.replace(/[A-Z]/g, shiftCipher);

    function shiftCipher(input) {
        return String.fromCharCode(65 + (input.charCodeAt(0) + 13) % 65 % 26);
    }

    console.log('RETURN => ', cipher);
    overlay(cipher);
    return cipher;
}