Add more Pandoc goodies

[alerque]

In working on a SILE Writer for Pandoc (discussion, PR) I keep bumping into things to tweak from the SILE side to match Pondoc's output. This is a running list of those tweaks, to be periodically added to master (so Pandoc testers can easily use it from Docker's siletypesetter/sile:master image), and at the very least merged when the writer is upstreamed into Pandoc master (and released to a stable build version before Pandoc support lands).

[alerque]

Merging "early" because the current SILE master branch does not match my current Pandoc fork output, and this branch does. I have a project going to press now, and the latest Pandoc + this matching module work together for it fine, older ones do not. I can build the project locally but I want it to build in CI before going to press.