Feature: Add HTML as mode bar download option

[mneilly]

Currently, the mode bar download button can support png, svg, jpeg and webp as options to toImageButtonOptions. There is also fig.write_html() which can be used too save an interactive html file.

It would be useful to support html for the mode bar download button to avoid the necessity of adding a dcc.Download explicitly and keep to a consistent interface... Or, alternately, allow the user to provide custom behavior.