use lingua-en-inflect to handle plurals in wordlist

Addresses issue #3.

This does not (yet) remove plurals from the word list.  If you like this approach, I can rebase it after the stopwords branch and strip the plurals out of that combined list.

I also see some other cruft that could be cleaned up, but I'll hold off until you merge the existing branches so we don't have too many things with conflicts.