[vim] fix Ctrl-[ (escape) shortcut for visual mode

This fixes a bug where Ctrl-[ does not leave vim visual mode.

Steps to reproduce:
1. go to http://codemirror.net/demo/vim.html
2. in the editor there, make a visual selection (type Vjj, for example)
3. Ctrl-[ should work like Escape, and cancel the visual selection, but it doesn't.

Escape does the right thing here, and Ctrl-[ works great to leave vim insert
mode. This is a one line patch to make Ctrl-[ work in visual mode