* 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.
docbook-xsl, a set of XSLT scripts to generate HMTL out of DocBook,
tries harder to generate a nice xref text. Depending on the element
being linked to, it looks at the title or other descriptive child
elements. Let's do that, too.
'xref' is used to create cross references to other parts of the
document. It is an empty element - the cross reference text depends on
various attributes. Quoting 'DocBook: The Definitive Guide':
1. If the endterm attribute is specified on xref, the content of the
element pointed to by endterm will be used as the text of the
cross-reference.
2. Otherwise, if the object pointed to has a specified XRefLabel, the
content of that attribute will be used as the cross-reference text.
