This saves about 20 pages (or 5%). Most figures improve with it, with a
couple of exceptions, where the base material is of low resolution already.
There is no mechanism yet to prescribe image width in markdown or
pandoc, cf. jgm/pandoc#261.
* Bumped version to 1.16.
* Added Attr field to Link and Image.
* Added `common_link_attributes` extension.
* Updated readers for link attributes.
* Updated writers for link attributes.
* Updated tests
* Updated stack.yaml to build against unreleased versions of
pandoc-types and texmath.
* Fixed various compiler warnings.
* Relative (percentage) image widths in docx writer.
* ODT/OpenDocument writer (untested, same issue about percentage widths).
* Update pandoc-citeproc.