Add warning for images not supported by browsers
Word allows some embedded image formats that are not supported by web browsers (e.g.
When XSweet encounters exotic image formats like this, it may be worthwhile to add some kind of warning, be it:
- a warning to the STDOUT for the processor
- an inline comment in the HTML
- perhaps even a literal DOM element warning
See xpub/xpub-epmc#63 for an example.