anchor.diff:  * reorg Prawn extensions (see prawn-table for example)
   * rename "theme" to "style"?
   * restrict custom theme path to jail (or load from load_path)
  -* add Optimizer class; wire to cli
   * implement convert_toc
   * can get orphan conum if starts on last line of page (fixed already?)
   * only create title page if doctype=book
  
positive.code: * remove remaining uses of vertical_rhythm and horizontal_rhythm
  * implement subtitle on title page
  * treat title-logo like the front-cover-image and back-cover-image (parse image macro)
  * theme setting for code line height (currently using base_line_height)
    - in general, allow theme_font to set line_height
  * 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)
  
negative.code: * list bullet in ordered list needs to grow with length of number
  * document how the treetop parser is rebuilt
  * rewrite optimize-pdf using rghost
    - add Optimizer class; wire to cli
  * rework font so we can set actual height, calculate x_height internally (use 1em for spacings)
  * padding top and bottom on content affects height_of calculations (need to review)
  * enable fallback font (Golo guide has problem with License block)
  