Add option to only select from books a user owns

[sillygwailo]

A Goodreads user can mark a book as owned. This may be information that's included in the book object, and if not, the API has a method to list books owned. Selecting from only owned books would make it easier to choose which book to read right now (especially if it's on the to-read shelf), rather than have to do something to get a copy of it.