Yi.Keymap.Vim now exposes exEval and exMode

exEval enables user to map events to ex commands like this:

 -- pressing space in normal mode resets search highlight
 (char ' ' ?>>! exEval self ":nohlsearch\n")

And exMode allows mapping an event to partial ex input, like this:

 -- this helps when you don't press Shift hard enough
 (char ';' ?>>! exMode self ":")