This allows users to turn off the default pandoc behavior of
parsing contents of div and span tags in markdown and HTML
as native pandoc Div blocks and Span inlines.
Setting of default epub extensions has been moved from the EPUB
reader to Text.Pandoc.
We now maintain the invariant that when fetchImages is called,
all images have absolute paths.
This patch fixes several bugs relating to this as there are three places
where images can be introduced.
(1) During the HTML parse
(2) As spine elements
(3) As a cover image
For (1), the paths are corrected by the transformation renameImages
For (2) and (3), we need to append the "root" to the path we parse from the
spine
Before the images were relative to the position of the package file. The
collapse function changed this so that they were then absolute in the
archive but the fetchImages function wasn't updated to recognise this.
