Referenced image not inserted in HTML and PDF exports (but well displayed by `bs serve`)

[vincent-ferotin]

When referencing an image, i.e. not inlining it, with ![Alt text][img-id] and then [img-id]: /img/url (see reference), desired image is not always well inserted in output medium -- depending on medium/backslide command:

Medium via command	Inserted or not
bs serve (browser)	ok, :-)
bs export (dist/HTML)	ko, :-(
bs pdf (pdf/PDF)	ko, :-(

See #64 for initial prospection and a demonstrating repository: https://github.com/vincent-ferotin/backslide-imageinpdf

[sinedied]

I think the image replacement regexp was never made to support image references.

As I said in your other issue, If you're willing to put up a PR to fix it, I'll gladly merge it but I don't have time to take a look at it myself anytime soon.