add book club support

[brianpipa]

Allow members of a book club to give their TOREAD list URL then OverReader can determine which books are the most common among them
One relatively easy way to do this is to have a page on the site where you enter multiple Goodreads shelves and it shows you the most common books. Could be useful in other use cases I haven't thought of yet.

[brianpipa]

1. user enters each TBR goodreads list in a separate line in a form on OverReader (this makes it easy for you to rerun it at a later date)
2. OverReader gets the books from each list and keeps track of how many times each book shows up
3. OverReader displays a page with the results sorted from most shelved to least