anchor.diff:  * support horizontal alignment of AsciiDoc table cell content (need to handle explicitly)
    ** only relevant when using subtable since it must have width < cell width
   
  +* format code to align with project standards
   * -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
  
positive.code:  ** required keys can never have a null value; most are set by base theme
  * allow font properties to be set for normal paragraph separate from base (need to think about inheritence)
  * allow alignment of list to be set separately from base align (and perhaps a hint in document) (#182)
  * FIXME: stop using fallback fonts in default theme (instead, bundle a fuller font)
   ** using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
   ** add broader character range to monospace font so we can drop fallback font by default (#282)
   ** consider having a fallback for prose and fallback for literal
  * use <a id=""></a> instead of <a name=""></a> for anchor point in formatted text
   ** benchmark to see if it's faster to use empty or non-empty element in parser
  * text decoration should be supported as part of theme_font
  
negative.code: 
  * QUESTION should we show keys with hyphens instead of underscores in theming guide?
  
  * fix theming guide regarding table striping
  
  * part/chapter/section start pages are calculated twice, once for each running content; should be shared
   ** probably need to introduce a model object to hold the cache
  * UPSTREAM: highlighting of CSS w/ rouge seems messed up; don't recognize keywords
   ** compare with pygments
   ** rouge also incorrectly identifies an endline in a schemaLocation attribute as an error
  * FIXME: stop using fallback fonts in default theme (instead, bundle a fuller font)
   ** using fallback fonts significantly slows down Prawn because it checks every letter every time (see https://github.com/prawnpdf/prawn/blob/master/lib/prawn/text/formatted/box.rb#L427-L434)
   ** add broader character range to monospace font so we can drop fallback font by default (#282)
   ** consider having a fallback for prose and fallback for literal
  * should we drop "<font>" handler in formatted text transform?
  * implement text transform for running content (#702)
  * implement start line number in source listing (rouge) (#752)
   ** shouldn't this be fixed in core at the same time? (core #1742)
  * implement line highlighting in rouge (#681)
  * only nest outline inside of root if doctype=article (still valid?)
  * warn if image in running content can't be found (#731)
  