anchor.diff:  * introduce theme keys for styling index
   * index term that wraps should have a hanging indent (#645)
   * rename inherited_align to text_align? (and base_align to base_text_align?)
  +* make sure any state is cleared after conversion, including attributes assigned
   
   * allow front cover and back cover image to be defined in theme as fallback; document in theming-guide
  +* keep together lines of a colist item?
   * FILE ISSUE only nest document outline inside root node if doctype=article
   * don't crash if image in running content is an unsupported format
   * support image URL (using resolve_image_path) in running content (what about data-uri?)
  
positive.code: * margins/paddings at page boundaries are strange, fragile
  * implement margin collapsing (between blocks)
    - would eliminate need for negative padding for blockquote
  * bw theme for CodeRay (or Rouge) to match output of Pygments bw
  * the nested? method on list isn't checking if nested inside a block inside a list
    - need an example
  * wrapped lines in source listing should be indented to account for line number gutter (#504)
  
negative.code: * support URL images in running content (need to delegate to resolve_image_path)
  * rewrite optimize-pdf using rghost (#535) (also see #21 and #22)
    - add Optimizer class; wire to cli (separate issue?)
    - we could recommend using HexaPDF with some sample code in README, but we can't integrate it since it's AGPL
  * add feature to number bullets according to section number (needed for OpenDevise agreements)
  * outline should link to title page if there's a cover page (skip cover page and ensuing blank page)
  * don't allow title page content to jump to next page
  