anchor.diff:    - remove unused theme / font files
   * theme keys should be flatted to arbitrary depth
   * enable line above (or below?) title on title page (file issue)
  -* quote all color values in reference themes
   * text_transform for table foot row
   * file upstream issue for Prawn to warn if it can't resolve a glpyh (or monkeypatch it)
   * make use of shrink_to_fit where appropriate
  
positive.code: 
  * create utility method to get % offset of page as y value (option to constrain to bounds)
  
  * document converter assignment in convert_content_for_block method
  * support transparency for colors
  * support generic color (or value) attribute in formatted text parser instead of specific color systems (rgb, cmyk)
  
negative.code: * introduce setting to indent section content
  * design merge margin logic (like for admonition block)
  * rename default theme to docbook theme, make default the Asciidoctor theme (should we have a base theme?)
  * allow relative font size for inline code to be set (perhaps a percentage or em value? there are problems with this in arranger)
  * set defaults in ThemeLoader for required theme settings like prose_margin_top/bottom so we don't need fallbacks in code
  * implement orphan sentences for paragraph
  