anchor.diff:  * separate theme control for listing vs literal block (and maybe source too)
   * allow title page image "bottom" to be set instead of "top" (mutually exclusive)
   
  -* scale block images like inline images
   * rewrite optimize-pdf using rghost
     - add Optimizer class; wire to cli
  +* layout SVG without using keep_together (since we know all the dimensions)
   * pass macro doesn't work in source block when macro subs and highlighting are both enabled
   * bind image_registry between scratch and main document so we don't process the same image more than once
     - need to do some testing
  
positive.code: 
  == TODO
  
  * upgrade Treetop
  * rename example to basic-example and chronicles to chronicles-example
  * recognize inline image syntax in page_background_image key
  
negative.code: * enable text_transform for table foot row
  * file upstream issue for Prawn to warn if it can't resolve a glpyh (or monkeypatch it)
  * support web fonts; use uri-cache to avoid redundant fetching
  * align caption to match alignment of block image
  * allow pdf-page-layout (portrait || landscape) to be set in document
  * allow pdf-page-margin to be set in document
  * attribute or role to control table shading (all, even, odd) (or call it striped like bootstrap?)
  * make conum glyphs configurable in theme (use reference table to resolve)
  * do we still need the converter hack in convert_content_for_block? (seems to be needed for admonitions)
  