anchor.diff:  * use padding from theme around block quote content
   * add admonition_label_font_color to theme
   * remove remaining uses of vertical_rhythm and horizontal_rhythm
  -* implement subtitle on title page
  +* 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)
  
positive.code: * theme idea / tester: see sandbox/ebook-learn_version_control_with_git-SAMPLE.pdf
  * 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
  
negative.code: 
  * source highlighting (with Pygments) not working inside an admonition block (see chronicles.adoc)
    - *don't need to run source highlighter in dry run*
  * getting a line wrap break before comma if preceding word is emphasized (problem in Prawn wrapping)
  * don't create blank page first page if content overflows and we're on the first page (already fixed?)
  * toc
    - make dot leader style separate from title / number
  * running content
  