* 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 are based off the reader tests, with some removed (where the
reader output was identical, based on different docx inputs). There
are still more to be added. In particular, tests for custom-styles
need to be added.
All golden docx files have been checked in MS Word
2013 (windows). There is no corruption.
There is questionable output in the `tables` test: the three tables
seemed to be joined. This will be addressed in a future commit, and
the golden docx file will be changed.
