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.