The default is `-raw_tex`, so no raw tex should result
unless we explicitly say `+raw_tex`. Previously some
raw commands did make it through.
Closes#4527.
* Deprecate `--strip-empty-paragraphs` option. Instead we now
use an `empty_paragraphs` extension that can be enabled on
the reader or writer. By default, disabled.
* Add `Ext_empty_paragraphs` constructor to `Extension`.
* Revert "Docx reader: don't strip out empty paragraphs."
This reverts commit d6c58eb836.
* Implement `empty_paragraphs` extension in docx reader and writer,
opendocument writer, html reader and writer.
* Add tests for `empty_paragraphs` extension.
