anchor.diff:  * add source language to upper-right corner of listing block
   * implement quote style from default Asciidoctor stylesheet
   * reorganize Prawn extensions (see prawn-table for example)
  -* rename "theme" to "style"?
  +* rename "theme" to "style"? (or allow both?)
   * restrict custom theme path to jail (or load from load_path)
   * enforce jail on SVG option enable_file_requests_with_root
   * implement convert_toc
  
positive.code: == Prawn Wishlist
  
  * fragment should be able to specify it's own width
  
  == Known Issues
  
  
negative.code:  ** 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)
   ** add broader character range to monospace font so we can drop fallback font by default (#282)
   ** consider having a fallback for prose and fallback for literal
  * transform table cell text inside correct font context (allows us to check the current font, such as in breakable_uri)
  * should we drop "<font>" handler in formatted text transform?
  * ttfunk 1.5.0 chokes on narrow no-break space char \u202f (patch applied; https://github.com/prawnpdf/ttfunk/issues/39)
  * implement text transform for running content (#702)
  * implement start line number in source listing (rouge) (#752)
   ** shouldn't this be fixed in core at the same time? (core #1742)
  