Add exitInsertMode, allow binding single keys only

This creates the command `exitInsertMode`. The command only responds to single key mappings, so that all keys except those which explicitly exit insert mode are passed to the underlying page.

This fixes #301.
At the moment, the implementation is a kind of poor-man's `completionKeys` for insert mode. In my mind, I was wanting #1315 to land, to implement modes mappings on it, and then integrate this logic there.

My vision is that the key handler could then just look like
```
(event) ->
  # Get a list of all the modes that currently apply (eg. find and normal; dialog showing and normal; find, dialog and insert; etc.)
  activeModes = getActiveModes()
  command = KeyHandler.getCommand event, activeModes
  executeCommand command if command.shouldExecute
  DomUtils.stopPropagation event if command.noPropagate
  unless command.passKey
    event.preventDefault()
    HandledKeys.push event
```
This implementation was merely a stop-gap.

I agree that your suggestion is much tidier, given the current state of the code.