anchor.diff:  * support !include in theme file (file issue)
   * margin per heading level (see https://github.com/asciidoctor/asciidoctor-pdf/issues/176)
   * add cover page example to chronicles so people see how to use it
  -* images
  -  - multiple image width by 0.75 to get point value (including in running header and footer)
  -  - should we use native size of image by default? (after multiplying by 0.75)
   * don't orphan a single line of paragraph (send it with a buddy line)
   * implement stem support
   * fail gracefully if theme file cannot be found
  +* expose theme variable on document (attr_reader?)
   * 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: 
  == TODO
  
  * upgrade Treetop
  * rename example to basic-example and chronicles to chronicles-example
  * recognize inline image syntax in page_background_image key
  
negative.code: * 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)
  
  * 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
  