All exporters should be able to resolve images

Currently, image references only work for the built-in exporters (PDF and EPUB). As the behavior currently stands, image references won't resolve when converting to HTML and DocBook (without some attribute hacking in the convert function). Requirement: Image and attachment references should work out of the box without any bespoke code.

The way to make this work is for image references in the assembled source to be consistent with how the converter works. It's possible that exporters will need a flag as to whether those references should be relative to the converted document (required for HTML) or relative to the docdir (required by EPUB).