Hide cursor during window selection process.

First of all, I have to express how much I love this package. Surely one of my most used functions and I couldn't live without it. Thanks for your work on this!

As for the pull-request, I'm not sure if this is something you're interested in so no worries if you'd rather not accept this change, but I started thinking the visual display of the cursor next to the numbers during the window selection process was kind of un-needed and unsightly. Originally I used my own defun to hide the cursor before and after `switch-window`, but thought you may be interested in having this change as part of the package.

Here's a visual example of this change:

![switch-window sans cursors](http://i.imgur.com/AadLC3f.png)

The user's original `cursor-type` is restored after the window configuration is restored.

Thanks again!