62668410 pdf png

This PR supports PDF images in the raw source, which look good in
PDF document, but need to be web-friendly in the HTML. We standardize
on PNG for simplicity. This means that, to do something like
    \image{images/foo.pdf}
authors need to have both `foo.pdf` and `foo.png` in their `images/`
directory. In this case, `foo.pdf` will be used in the PDF output, while
`foo.png` will automatically be used in the HTML, EPUB, & MOBI versions.