Vim: add "j" to 'formatoptions'

Stolen from @lencioni:

https://github.com/lencioni/dotfiles/commit/03bc22291dc

'formatoptions' is an interesting setting, because it is buffer-local in
nature, but any options you add during startup wind up in the default
list. So, I already had "tcq" (the Vim defaults) plus "n" (which I'd
added). Most filetype plugins add to this list on a per-buffer level
(adding useful things like "l", for example), but curiously, "j" isn't
one that gets added (seems like a sane default, so not sure why).

Anyway, as I said to @lencioni via chat, this is going to save me
approximately 1,000 seconds per year.