* 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
These were never added when the tests were first created.
Output files checked in MS PowerPoint 2013 (Windows 10, VBox). No
corruption, and output as expected.
