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: * UNRESOLVED: dry_run should start at cursor of main document (or as option); total height calculation would need to be revised
    - box_height isn't currently accurate when it spans more than one page
    - this should fix height calculation when content is split over a page break (leaving small amount of excess)
    - however, if cursor is advanced to fit content on page, then that excess will cause box_height to be too large
    - life would be simpler if Prawn allowed us to draw graphics at bottom layer
  * support pdfwidth for inline image (#620)
  
negative.code: 
  == TODO
  
  * use background color from pygments theme for source block
  * what should we set as the border color if the source highlighting theme has a background color?
  * height of listing block not calculated correctly when string of contiguous characters exceeds length of line
    - put listing block with very long line inside of sidebar; see that sidebar height is incorrect (too large)
    - is this because source highlighting is not done in scratch document?
  * backport resize method from prawn-svg and use it in converter (now in two places)
  * rework resolve_image_path API so it's more logical; override based on type of first argument; document as option
  * rename text-alignment attribute to text-align? (change is within alpha.14, so still a chance to change)
  