Allow setting reference-docx in YAML front matter.

[jtkiley]

I often use the same few reference-docx documents for a number of documents that I convert. It would be really nice to set it in the YAML front matter, toward a goal of being able to covert documents within my text editor (Atom) without needing to specify a bunch of settings on the command line. There are probably other variables that would need to be settable in YAML to make that happen, but paths to other files seem to be one of the more time-consuming things.

Currently, I use a Makefile with the command in it, but I have to set up a target for every new file in any given project. With this change in place (and perhaps others), I could just use a snippet for new documents.

I'm not sure what the logic is for variables that are or are not settable in YAML, but it would be nice for every option available on the command line to be there. That would make in text editor automation really easy for a package to implement.

[mb21]

Closing in favour of #4627.

[VolkovIlia]

May you write a little example of usage reference-docx in YAML?