anchor.diff:  * honor safe mode rules
   * allow cover images to be specified by theme as a fallback
   * verify cover image exists; fail gracefully with warning
  -* using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
  +* stop using fallback fonts in default theme (instead, bundle a fuller font)
  +  - using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
   * print scratch.pdf file if verbose / trace mode is on in Asciidoctor
   * introduce setting to indent section content
   * design merge margin logic (like for admonition block)
  
positive.code: 
  == TODO
  
  * recognize inline image syntax in page_background_image key
  * allow front cover and back cover image to be defined in theme; document in theming-guide
  * keep caption with table (check for sufficient space)
  
negative.code:   - 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
  * dedicated style for top/bottom margin of outline list
    - allow margin top and bottom to be set for lists (applies to outer-most list)
  