convert to htmlwidgets version

[`htmlwidgets`](http://www.htmlwidgets.org) provides the infrastructure to easily bridge `R` with `JavaScript`.  Utilizing this infrastructure allows use of our `mermaid` diagrams in Shiny, in the R console, in R markdown, and in @rstudio viewer.  This pull also might help others considering converting their existing R <-> JS packages into `htmlwidgets`.

References issue #1 insuring that @rich-iannone would be ok with the premise of converting