missing-letters
Find the missing letter in the passed letter range and return it.
If all letters are present in the range, return undefined.
#Tests
fearNotLetter("abce") should return the string d.
fearNotLetter("abcdefghjklmno") should return the string i.
fearNotLetter("stvwx") should return the string u.
fearNotLetter("bcdf") should return the string e.
fearNotLetter("abcdefghijklmnopqrstuvwxyz") should return undefined.