* 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.
Closes#261.
TODO:
* Relative (percentage) image widths in docx writer.
* ODT/OpenDocument writer (untested, same issue about percentage widths).
* Update pandoc-citeproc.
Replaced styles.xml in headers.docx with pandoc's current styles.xml, which
contains styles for Heading 1 through 6. Added Heading 4 through 7 to the test
document. Note that Heading 7 is not parsed as a Heading because there is no
Heading 7 style.
Renamed some tests, introducing subsidiary directories
for fb2, docx, epub.
Cleaned up tests in cabal file.
Combined dokuwiki-writer and dokuwiki_inline_formatting tests.