vim: make the 81st column stand out

From @thoughtstream/Damian-Conway-s-Vim-Setup
vim: Make tabs, trailing whitespace, and non-breaking spaces visible

From @thoughtstream/Damian-Conway-s-Vim-Setup
vim: open any file with a pre-existing swapfile in readonly mode

From @thoughtstream/Damian-Conway-s-Vim-Setup
vim: refactor NoSimultaneousEdits