anchor.diff:  * autofit logic not working with Courier (still overrunning line)
   * honor safe mode rules
   * allow cover images to be specified by theme as a fallback
  -* verify cover image exists; fail gracefully with warning
   * 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)
   * print scratch.pdf file if verbose / trace mode is on in Asciidoctor
  
positive.code: 
  == TODO
  
  * SIMPLE: use literal cell style for theme-related document attributes section of theming guide
  * dry_run should start at cursor of main document (or as option); total height calculation would need to be revised
    - this should fix height calculation when content is split over a page break (leaving small amount of excess)
  * FILE ISSUE: prevent import_page from adding an extra page; leaves blank page when placing two inserts in a row
    - have import_page first delete current page if page is empty
  * FILE ISSUE: if the image type cannot be resolved from file extension (e.g., when target is a URL), honor format attribute like core
  * SIMPLE: add warning that AsciiDoc table cell content is not supported; print as raw source
  * cache stateless cell data resolved from theme (don't need to recalc each time; at least per table)
  * autowidth on table doesn't work for multi-line content (width based on normalized value)
  * table logic: does the layout_table_caption have to be inside the table block? can we pre-calculate the actual width for the caption? does the table offer a callback we can use to keep the caption on the same page as the table?
  * FILE ISSUE: enable/implement key table_font_style
  * introduce object to store/organize running content data and specs
  * QUESTION: should we set pdf-anchor attribute on every node that has an id?
  * QUESTION: should theme font handle hierarchical keys (either explicitly or implicitly)
  * need to support .canvas role on image so it isn't shrunk to fit inside top/bottom margins
    - perhaps .canvas, .canvas-x, .canvas-y
  * FILE ISSUE: when split source listing, add top padding to bounding box (or is it the line metrics top?)
  * prawn-gmagick (fixes PNG transparency issue, adds gif support); don't fail-fast on gif if gem is detected
    - https://github.com/packetmonkey/prawn-gmagick
  * stroke and fill multi-page sidebar block (#259) and example block (#362)
  
negative.code: * make source code highlighting theme configurable (should be now, but has problems with conums); still?
  * use or don't use pad method? check performance
  
  == Major Milestones
  
  * add a test suite
  * refactor as Prawn View
  * add support for AsciiDoc table cell content by writing custom Table::Cell implementation
  * add support for nested tables by writing custom Table::Cell implementation
  * add support for footnotes (as article or chapter endnotes)
  * rework text handling in Prawn to support line height natively
  
  == Documentation
  
  * control page numbering using pagenums attribute
  