anchor.diff:  * document how the treetop parser is rebuilt
   * rewrite optimize-pdf using rghost
     - add Optimizer class; wire to cli
  +* use ImageMagick to uncompress PNG images before reading them (could also just document this)
   * 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)
  
positive.code: * do we still need the converter hack in convert_content_for_block? (seems to be needed for admonitions)
  * avoid getting an empty last page (example: colist at bottom of page can cause this)
  * utility to coerce the color value transparent to nil (better handling in general)
  * cjk and/or multilingual support
  * description list term should keep together with content (file issue)
  * GhostScript Tiger SVG is reporting incorrect height when embedded (too much spacing below); bug in prawn-svg?
  * hardbreak in table cell results in extra endline (likely not normalizing cell content)
  
negative.code: * make alternating page title position optional (via theme?)
  * BUG: page numbers are off in Clojure Cookbook
  * image in header / banner (need to implement a masthead)
  * fix passthrough placeholders that get caught up in syntax highlighting (see https://github.com/asciidoctor/asciidoctor/blob/master/test/blocks_test.rb#L2258-L2277)
  * we could eliminate some of the tags we're currently matching in the formatted text parser (e.g., link)
  * add Preamble to TOC
  