amend readme

amend readme

update for pandoc 2.3.1

doc: update

update to fix https://github.com/msprev/panzer/issues/45

fix: don't clobber additative metadata fields with in_doc's metadata

improve info messages

improve documentation in response to https://github.com/msprev/panzer/issues/38

improve info message formatting

change implementation of lua filter support

run lua filters as part of pandoc write pass

update documentation

update doc

implement lua filters

call out to external processes using `shell=True` to address issue under Windows https://github.com/msprev/panzer/issues/36