Write the function
common_word that takes the file path to a text file (.txt) and an integer
n and returns nth common word in the text file. Assume the words in the string are separated by white-space and they are case-insensitive. If multiple words are identified as the nth common word, they must be returned as a single string sorted alphabetically in reverse order and comma separated.
filepath = 'jim-rohn.txt' common_word(filepath, 2) -> 'small' common_word(filepath, 1) -> 'you.,others,not,let,but'