[ACS] Attach supplementary files. Add hidden pref to attach high res PDF

Re https://forums.zotero.org/discussion/25844/acs-pdf-fulltext-problem/

I'm on the fence about the preferred PDF attachment. Right now it's high-res, but I think @adam3smith is right in that the figures are legible... they're just blurry and could be annoying to read or look bad if you're trying to copy them for a presentation. Probably space is a bigger concern for most people, so maybe it should stay as is. Either way, with the hidden pref, everyone can be happy. (hidden pref: 0 = whatever the default is, 1 = the other option, 2 = both)

I did forget to comment the code. I'll do it in the morning.