anchor.diff:  * implement stem support
   * fail gracefully if theme file cannot be found
   * expose theme variable on document (attr_reader?)
  -* nested unordered list items should use different marker
   * dedicated style for top/bottom margin of outline list
     - allow margin top and bottom to be set for lists (applies to outer-most list)
     - allow spacing between nested lists levels be configured in theme
  
positive.code: * font families used in SVGs must match keys in the font catalog
  
  == Open Questions
  
  === Design
  
  
negative.code:   - also combine with the preserve_space logic
  * allow front cover and back cover image to be defined in theme; document in theming-guide
  * should we shorten the keys to front-cover and back-cover (since image is implied?)
  * keep caption with table (check for sufficient space); only for top placement since bottom placement is much harder
  * allow valign to be set on image block (vertical center in page for things like slides)
  * 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)
  * rtl (see ./sandbox/rtl/ folder)
  
  * pass macro doesn't work in source block when macro subs and highlighting are both enabled (#180)
  * enable cache_images option for prawn-svg (#223)
  * bind image_registry between scratch and main document so we don't process the same image more than once
    - need to do some testing
  * show SVG warnings if debug (or trace) is on
  * a nested image with a relative path in an SVG is resolved relative to the SVG source file (document in README)
  * clean temporary files once per conversion? (file issue)
  * clear font paths in SVG interface so it doesn't scan system? (since it's not portable anyway)
  * title is being rendered 3 times (maybe one for scratch?); explain why in comments if normal
  