...in numbering.xml. This caused pandoc-produced docx files to
be uneditable using Word Online.
The problem was that recent versions of reference.docx include
samples of various kinds of text, including lists. The
numering elements for these were getting copied over to
the new docx, where they clashed with the autogenerated
elements produced by pandoc. This didn't confuse Desktop
Word, but it did confuse Word Online.
Closes#5358.
* docx writer: support custom properties. Solves the writer part of #3024.
Also supports additional core properties: `subject`, `lang`, `category`,
`description`.
* odt writer: improve standard properties, including the following core properties:
`generator` (Pandoc/VERSION), `description`, `subject`, `keywords`,
`initial-creator` (from authors), `creation-date` (actual creation date).
Also fix date.
* pptx writer: support custom properties. Also supports additional core
properties: `subject`, `category`, `description`.
* Includes golden tests.
* MANUAL: document metadata support for docx, odt, pptx writers
