This adds a workaround to a bug in the interaction of the
markdown and knitr packages that caused the vignettes
to attempt to produce snapshots if Pandoc was not available,
even though Pandoc is not needed.
This small release is at the request of Tomas to adapt to upcoming changes in Rtools on Windows. It also contains a number of other changes (mostly bug fixes and minor improvements, described in the NEWS.md file). I ran revdep checks and didn't see any problems.