Fix the E selection bug

I added the `E` motion before, but find a selection bug these days.

The bug is that when you're in selection state and you press `E` more than once, then selection will be reset.

btw, I'm not sure if there's a method provided by Atom to get the next position, so I just use this:
```coffeescript
{row, column} = @editor.getCursorBufferPosition()
start = new Point(row, column + 1)
```

Is this OK?