anchor.diff:  * HIGH: style example block
   * HIGH: allow theme to be customized using stylesheet attribute
   * MEDIUM: headshot has too much top margin when at top of page (not below section title)
  -* MEDIUM: add landmarks nav list to nav.xhtml
   * support both jpg and png avatars
   * add docinfo support
   * add navigation links in meta, e.g.,
  
positive.code: //^
  
  .Packager
  * HIGH: add NOTICE.adoc to e-book archive
  * HIGH: rework GEPUB so closures work more like in Prawn...perhaps using a wrapper
  * HIGH: minimize CSS (and any other assets worth minimizing)
  * HIGH: add back cover if specified
  * MEDIUM: set modified date explicitly
  * MEDIUM: use function to build and manipulate image paths
  * MEDIUM: issue warning if front-cover-image doesn't match proper syntax
  * support subtitle as separate from main title in package metadata
  * support collection title in package metadata
  
  
negative.code: * HIGH: use more thorough image path resolution
  * HIGH: move navigation_document method to Packager
  * HIGH: put shy marks after dashes to allow wrapping to occur in text justification
  * MEDIUM: add landmarks nav list to nav.xhtml; see https://github.com/IDPF/epub3-samples/blob/master/31/moby-dick-mo-xhtml/EPUB/toc.xhtml
  * MEDIUM: Kindle links to cover page when clicking on first chapter instead of first chapter page
  * MEDIUM: auto-space button in btn macro, or add icon & icon placement as attributes on btn macro
  * MEDIUM: move client sniffer JavaScript to file
  * MEDIUM: use the dfn element for defining a definition of a term (not just term class); see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dfn
  