anchor.diff:  
   == TODO
   
  +* replace explicit char ranges with classes in regexp (e.g., [[:word:]] or \w)
   * upgrade prawn-svg to 0.22.1 (keep in mind minimum Ruby version is 2.0)
   * test Rouge upgrade (should address workaround we have in encoder)
   * prawn-gmagick (fixes PNG transparency issue); don't fail-fast on gif if gem is detected
  
positive.code: * 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)
  
  * rewrite optimize-pdf using rghost
    - add Optimizer class; wire to cli
  
negative.code: * support arbitrary numeric entities (convert to unicode)
  * add brief mention in theming guide that deeper customizations can be achieved by extending the converter
  * allow alignment of list to be set separately from base align (and perhaps a hint in document)
  * due to order of loading, getting warnings when redefining methods in Asciidoctor core
  * prawn-pdfimage https://github.com/packetmonkey/prawn-pdfimage
  * rename ThemeLoader to ThemeReader?
  * document autofit option on verbatim blocks
  
  * *margin per heading level* (see https://github.com/asciidoctor/asciidoctor-pdf/issues/176)
  * document how to override the Ruby code to get custom styling in the theming guide
  