anchor.diff:  * file upstream issue for Prawn to warn if it can't resolve a glpyh (or monkeypatch it)
   * support web fonts; use uri-cache to avoid redundant fetching
   * align caption to match alignment of block image
  -* allow pdf-page-layout (portrait || landscape) to be set in document
  -* allow pdf-page-margin to be set in document
   * attribute or role to control table shading (all, even, odd) (or call it striped like bootstrap?)
   * make conum glyphs configurable in theme (use reference table to resolve)
   * 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)
  
positive.code: == Potential Optimizations
  
  * if autofit is set on a listing/literal block that has conums, we are splitting fragments by line twice
  
  == Usage Optimizations
  
  
negative.code:   - perhaps progit2 as well; others?
  
  == TODO
  
  * recto/verso margins
    - account differences in top/bottom margins on recto/verso pages in running header/footer
  