Allow html in labels
Allow html in labels

Fixes #817
jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets