To make loading of extras/ addons simpler, and remove cruft from
presentation html files, I created a new plugin src/plugins/extras/.
If any of the extra addons (highlight.js, markdown.js, mathjax.js
or mermaid.js) are added to the html file with a regular <script> tag,
then this module will discover and know how to init the module for you,
so it is not necessary to do that in html. If you're not using
the extras, this plugin does nothing.
Note that in the branch history where you are reading this commit,
the extras/ directory doesn't actually exist yet, nor do the
examples/ that use them. But they will hopefully join this branch soon.
