Continuelist: Hitting enter twice cancels the list

First mentioned in https://github.com/codemirror/CodeMirror/issues/2621#issuecomment-49618512. 
This mimicks the behavior of other popular text editor.

![http://g.recordit.co/VNsQRWAZ8N.gif](http://g.recordit.co/VNsQRWAZ8N.gif)