Show Tree View when opening a folder path

When opening a folder path, it seems logical that the tree view should always show since the usual next step would be to open a file within that folder. Showing it automatically saves a step of manually toggling the tree view open. Atom already auto-showed the tree view when opening a folder path in a new window, but it wasn't when opening a folder path in an existing window.

![show-tree-view](https://cloud.githubusercontent.com/assets/944302/4080588/fc77024a-2edf-11e4-8c5d-4f80c7f156b7.gif)