anchor.diff:  * allow default kerning to be set using theme
   * keep line comment in front of callout number to aid copying?
   * rework pull request for source line numbers (combine with restore conum logic if conums are enabled)
  -  - also combine with the preserve_space logic
  + ** also combine with the preserve_space logic
   * should we shorten the keys to front-cover and back-cover (since image is implied?)
   * keep caption with table (check for sufficient space); only for top placement since bottom placement is much harder
   * allow valign to be set on image block (vertical center in page for things like slides)
  
positive.code: * Should the heading sizes be calculated according to the default font size?
  * Page margins
  * Body indentation?
    - recto / verso indentation?
  * Size of masthead / footer
  * Line separating masthead / footer?
  * Separate title page
  
negative.code: * look into using close! on tmp file, which will safely unlink
  * justify does not work in normal table cell (and does not inherit from base)
  * support horizontal alignment of AsciiDoc table cell content (need to handle explicitly)
   ** only relevant when using subtable since it must have width < cell width
  
  * -v doesn't turn on warnings soon enough to catch warnings in Asciidoctor/Asciidoctor PDF
   ** might need to look ahead at arguments
  * 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?
  * support image URL (using resolve_image_path) in running content (what about data-uri?)
  * numbering for appendix subsections is not correct; should be A.1, A.2 (#627)
   ** seems like an issue in core too
  * BUG: http://asciidoctor.org[Asciidoctor] surrounded by smart quotes doesn't get translated to a link (as it should)
  * FILE ISSUE: draw border for quote/verse block on right if text is aligned to the right
  * Prawn should not move cursor before placing image if image exceeds height of bounding box
   ** ^ workaround in place by overriding move_text_position
  * part title / number (#597)
   ** upper roman numeral
   ** add part-label, fallback to Part (e.g., Part I)
   ** only use roman numeral in toc
  * support equal column widths in header/footer as "columns: 3*" (with optional leading alignment)
  * support padding for each column in running header/footer
  * should we move files under asciidoctor/pdf and use asciidoctor-pdf as the alias? (#262)
  