compliance with the new htmlwidgets convention

[stla]

From the htmlwidgets 1.5.3 NEWS:

Support a new PACKAGE::widget_html.WIDGETNAME convention for defining custom widget HTML. This replaces the earlier PACKAGE::WIDGETNAME_html convention, which continues to work but may be deprecated at some point in the future. The goal for the new convention is to prevent accidentally matching functions that were never intended for this purpose.

[timelyportfolio]

Thanks so much for this contribution @stla. I'll test and plan to merge soon. I will also update DESCRIPTION to require latest htmlwidgets.