* 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
Modify the PowerPoint tests to run all the tests with
template (--reference-doc) as well. Because there are so many
interlocking pieces, bugs can pop up in weird places when using
templates, since it changes how the writer builds its output
file.
For example, I recently discovered a bug in which speaker notes worked
fine and templating worked fine elsewhere, but templating with speaker
notes produced a file that would crash MS PowerPoint. That particular
bug was fixed, but this will forces us to check for that with each new
change.
