Expose treeView on atom.workspaceView

As seen in the status-bar package: http://git.io/Do6RPQ

This would allow for package authors to access, among other things, the currently selected file/directory in the tree: meaning package authors can create key bindings for path-specific commands, such as `npm init`.

If `tree-view` has been kept private intentionally, it would be great to still have a way to easily access to the currently selected file :) Thanks!