Add option to keep tree view while in zen mode

This is a change I did locally for my workflow and I figured I'd see if I could get it merged in upstream. Obviously most people want zen mode to have as few options as possible in it, but I do like the option to still open the tree view while in it. As it stands, it's a major hassle to have to leave zen mode to do one of my most common actions. 

I left the tree view as being hidden by default so that there would be absolutely no change to the end user unless desired.

![Preview](http://cl.ly/image/1c2I2w2G2N3c/zenview.gif)

This should probably not be merged in until after https://github.com/defunkt/zen/pull/6 as I built on top of that.