Only process standalone images into figures

[zkamvar]

This will fix #445 by only processing known figures.

That being said, it will be good to process the <img> tags that maintainers have added in because they wanted to control the width attributes.

TODO:

• make sure to add "figure" class to images so they have a max width of 100%
• process lonely image tags
• investigate failures of this on nightly pandoc (likely because the figure class is now "float"?)

  <div class="float" id="fig-first">
    <img src="fig/Setup.png" class="imgclass" alt="alt text" />
    <div class="figcaption">link should be transformed</div>
  </div>