Preview behavior (like Sublime preview)



Doesn't open a new tab every time the user click on a file item (like in Sublime), instead, it creates a atom.workspace.activePane.previewingItem, which will be removed if the user click on other file. It stays if the user doubleClick on the filename

![tab](https://f.cloud.github.com/assets/2464274/2292340/581f0c0e-a057-11e3-8198-ed1756f7b8f5.gif)