Notes on rglwidget processing:

Use cases:

At the command line through auto printing or explicit rglwidget
call, in RStudio or not.

In a knitr/rmarkdown document, through auto printing, an
rglwidget call, or by way of a hook function.

When should snapshots be converted to URIs? -- <img> links with
file source won't display properly in RStudio, but URIs do.
URIs don't display in Github Markdown as used in README (but
Pandoc knows that, so it's not a big deal:  put the filename
in the Markdown, and let Pandoc produce the HTML.)
