anchor.diff:  * rename inherited_align to text_align? (and base_align to base_text_align?)
   * make sure any state is cleared after conversion, including attributes assigned
   * when removing callout numbers, also remove the leading space so we don't mess up the highlighter
  -  - in particular, messing up highlighting for apache conf file
  + ** in particular, messing up highlighting for apache conf file
   * make dpi configurable (controls the px to pt conversion)
   * should we make :px the default units in to_pt? or require explicit units?
  -* use background color from pygments theme for source block
  -* cache background color of source highlighting theme (esp for pygments)
  +* QUESTION should we cache background color of source highlighting theme?
   * block anchors should be positioned on same page as content if content is forced to new page
  -  - call theme_margin <type>, :top first
  -  - theme_margin should return distance moved (0 if advanced to new page)
  -  - pass effective margin as optional argument to add_dest_for_block to adjust placement
  -  - already handled for block images
  + ** call theme_margin <type>, :top first
  + ** theme_margin should return distance moved (0 if advanced to new page)
  + ** pass effective margin as optional argument to add_dest_for_block to adjust placement
  + ** already handled for block images
   * QUESTION: should we skip starting new page if image doesn't fit on whole page?
   * consider using unlink on creation for tmpfile; see http://ruby-doc.org/stdlib-1.9.3/libdoc/tempfile/rdoc/Tempfile.html#method-i-unlink
   * might be faster to not use TextDecorationTable lookup in to_styles (though it's only called once)
  
positive.code: * 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
  * 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)
  
negative.code: * devise a way to specify a value as a string literal (variable replacement only) in theme
  * apply calculated theme values after loading?
  * allow "content" in place of recto_content & verso_content for running header/footer
   ** still relevant after restructuring?
  * be more specific in theming guide as to where prose_margin_top and prose_margin_bottom apply
  * allow valign value to be a number (requires change to Prawn)
  * allow background color to be set for chapter / heading
  * allow border to be set around block image
  * file issue in prawn to dispatch to image handler for images it doesn't know about
  * add brief mention in theming guide that deeper customizations can be achieved by extending the converter
   ** see sandbox/asciidoctor_pdf_extensions.rb
   ** reference infoq-minibook repo & blog post
   ** document how to extend the converter, use Prawn
   ** document how to override the Ruby code to get custom styling in the theming guide
  * is https://github.com/packetmonkey/prawn-pdfimage a safer way than prawn-templates to import PDF as image?
  * rename ThemeLoader to ThemeReader (or ThemeManager)?
  * normalize step leaves space after endline at a hard line break (doesn't seem to affect flow)
  * *margin per heading level* (#176)
  * need a single object to hold complete font properties; different from font family/style object
  * font method should support a single argument that's a font object or font hash
   ** in general, the way font properties are set needs to be cleaned up
  * allow font size in theme to be specified in em or %
   ** should multiply value being inherited
  * can't put margin top on chapter (chapter_top?) (#576)
  * convenience method to check if there's enough room for another line on page
  * allow dynamic background image with page number in path
  * running header/footer covers content (perhaps just a limitation that needs to be documented)
  * document that palette-based transparent in PNGs is not supported in older version of Prawn
  * FILE ISSUE: for prawn to preserve space (even w/ guards, spaces don't preserve over wrapped lines)
   ** if this is fixed, we can remove all the guard indent code
   ** we also have a problem that soft hyphens in wrapped content get dropped
  * FILE ISSUE: for prawn to support spacer fragments with fixed width / height and no text (or text is ignored in calculations)
   ** needed for arranging inline objects
  * document limitations in README (such as no linear gradients in SVG, etc)
  * document all permutations of image sizing
  * set vposition on title page logo image explicitly to avoid page overrun?
  