vimrc: Conditionalize "set fo+=j".

The "j" option was added in Vim 7.3.541:
ftp://ftp.vim.org/pub/vim/patches/7.3/README
tmux: Enable use of tmux-MacOSX-pasteboard.

I should probably conditionalize this at some point, based on whether
reattach-to-user-namespace exists in the usual places.