Follow updated Chicago title casing rules

[adam3smith]

The Chicago Manual has changed its title casing rules

Note that Chicago now recommends capitalizing prepositions of five letters or more (see rule 3).

I'd argue that we should update CSL's behavior accordingly since we've always followed the manual on title-casing. This is done with the stopword list at https://github.com/citation-style-language/schema (which I believe is then separately picked up by the citeprocs.

A more ambitious solution is something like #106 that would allow us to meet both Chicago 17 and prior and Chicago 18 compatibility, but that requires a significant schema update, whereas we can, I would argue, update the stop-word list without actually updating the schema version.

cc @adunning @bwiernik

[bwiernik]

I agree with a fast update to the stop word list and we can keep #106 for future

[bwiernik]

When we address #106, we can also do abbreviations lists