enable header/footer by default
fill in missing settings for default theme
port enhancements from editions branch

- support inline macro for cover image
- restore attributes after render
- use left alignment for bibliography entries, add bullets
- skip cover page if notitle is set
- support additional theme settings for headings
- introduce extension point delegates
- special handling for author bio (perhaps temporary)
- force link color
- prose around image
- first line formatting; apply to abstract block
- additional sanitizing for text
- colist numbers
- align and scale SVG properly
- enhancements for pygments highlighting
- custom page numbering pattern
- use publisher instead of producer attribute for producer field
fix alignment of admonition label
pass section to start_new_chapter; add comment
upgrade dependencies

- upgrade Prawn to 1.0.0
- upgrade Prawn-SVG to 0.15.0.0
- upgrade Treetop to 1.5.3
fix margins on sidebar title
update example output