Add stop-words.json

Updates the stop words that should be recognized by CSL processors, and places them in a separate JSON file.

See http://xbiblio-devel.2463403.n2.nabble.com/title-casing-skip-words-td7578629.html and https://gist.github.com/adam3smith/6326169 .

I'll accept this pull request once people have had a chance to review.