book icon for notebooks, color folders differently
book icon for notebooks, color folders differently

This allows us to use the generic file icon for text files, etc, down
the line.

cornhundred/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets