Added `self.window.folders()` fallback to _determine_path

This is kinda follow up to pull request #18.  When a new window is opened, the FileBrowser currently defaults to the user's home directory.  IMO, it'd be nice if the extension checked to see if any directories are available in `self.window.folders()` before defaulting to the user's home directory.  This is handy when sublime is opened from the command line.