zsh config - don't do XOFF on Ctrl-S

See: http://stackoverflow.com/questions/3446320/in-vim-how-to-map-save-to-ctrl-s
Vim config - Ctrl-S save and bring back I insert

- Ctrl-S saves in Normal Mode or Insert Mode.

- I bound `I` to add a newline above and stay in Normal Mode (like `O`
just doesn't switch into Insert Mode), totally forgetting that `I` was
something I use all the time to go into Insert Mode at the beginning of
a line.  Now `U` inserts a line above while staying in Normal Mode.