* Text.Pandoc.Options.WriterOptions: removed writerReferenceDocx
and writerReferenceODT, replaced them with writerReferenceDoc.
This can hold either an ODT or a Docx. In this way, writerReferenceDoc
is like writerTemplate, which can hold templates of different
formats. [API change]
* Removed `--reference-docx` and `--reference-odt` options.
* Added `--reference-doc` option.
This just tests whether a custom style with a recognizable
style (italic etc, defined in a reference.docx) will roundtrip back to
that format (i.e., whether `<span custom-style="Emphasized">` will
roundtrip to `Emph`). The custom styles are defined in the
`custom-style-reference.docx` included in the docx dir.
Sometimes we will want to get back something different than we started
with in a round-trip test. This allows for that, and makes the perfect
roundtrip a special case.
