Allow list arguments in default files to accumulate

[jgm]

For example, if defaults1.yaml is

include-in-header: A.txt

and defaults2.yaml is

include-in-header:
- B.txt
- C.txt

and the command line is

pandoc --include-in-header D.txt --defaults defaults1.yaml --defaults defaults2.yaml

then we should get four files included, in the order: D.txt, A.txt, B.txt, C.txt.

This affects many of the arguments in defaults files. Note that some of these are currently accumulated in reverse in option processing (when multiple options are allowed), and we may want to regularize this a bit.

We also need some tests for this sort of thing.

[jgm]

This may also help with #3139

[dhimmel]

What is the difference between --defaults and --metadata-file?

Update, I see the documentation at:

pandoc/MANUAL.txt

Lines 1421 to 1591 in 9c7f75a

	# Default files
	The `--defaults` option may be used to specify a package
	of options. Here is a sample defaults file demonstrating all of
	the fields that may be used:
	``` yaml
	from: markdown+emoji
	# reader: may be used instead of from:
	to: html5
	# writer: may be used instead of to:
	# leave blank for output to stdout:
	output-file:
	# leave blank for input from stdin:
	input-files:
	- preface.md
	- content.md
	# or you may use input-file: with a single value
	template: letter
	standalone: true
	self-contained: false
	# note that structured variables may be specified:
	variables:
	documentclass: book
	classoption:
	- twosides
	- draft
	# metadata values specified here are parsed as literal
	# string text, not markdown:
	metadata:
	author:
	- Sam Smith
	- Julie Liu
	metadata-files:
	- boilerplate.yaml
	# or you may use metadata-file: with a single value
	# Note that these take files, not their contents:
	include-before-body: []
	include-after-body: []
	include-in-header: []
	resource-path: ["."]
	# filters will be assumed to be lua filters if they have
	# the .lua extension, and json filters otherwise. But
	# the filter type can also be specified explicitly, as shown:
	filters:
	- pandoc-citeproc
	- wordcount.lua
	- type: json
	path: foo.lua
	file-scope: false
	data-dir:
	# ERROR, WARNING, or INFO
	verbosity: INFO
	log-file: log.json
	# citeproc, natbib, or biblatex
	cite-method: citeproc
	# part, chapter, section, or default:
	top-level-division: chapter
	abbreviations:
	pdf-engine: pdflatex
	pdf-engine-opts:
	- "-shell-escape"
	# you may also use pdf-engine-opt: with a single option
	# pdf-engine-opt: "-shell-escape"
	# auto, preserve, or none
	wrap: auto
	columns: 78
	dpi: 72
	extract-media: mediadir
	table-of-contents: true
	toc-depth: 2
	number-sections: false
	# a list of offsets at each heading level
	number-offset: [0,0,0,0,0,0]
	# toc: may also be used instead of table-of-contents:
	shift-heading-level-by: 1
	section-divs: true
	identifier-prefix: foo
	title-prefix: ""
	strip-empty-paragraphs: true
	# lf, crlf, or native
	eol: lf
	strip-comments: false
	indented-code-classes: []
	ascii: true
	default-image-extension: ".jpg"
	# either a style name of a style definition file:
	highlight-style: pygments
	syntax-definitions:
	- c.xml
	# or you may use syntax-definition: with a single value
	listings: false
	reference-doc: myref.docx
	# method is plain, webtex, gladtex, mathml, mathjax, katex
	# you may specify a url with webtex, mathjax, katex
	html-math-method:
	method: mathjax
	url: "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"
	# none, references, or javascript
	email-obfuscation: javascript
	tab-stop: 8
	preserve-tabs: true
	incremental: false
	slide-level: 2
	epub-subdirectory: EPUB
	epub-metadata: meta.xml
	epub-fonts:
	- foobar.otf
	epub-chapter-level: 1
	epub-cover-image: cover.jpg
	reference-links: true
	# block, section, or document
	reference-location: block
	setext-headers: true
	# accept, reject, or all
	track-changes: accept
	html-q-tags: false
	css:
	- site.css
	# none, all, or best
	ipynb-output: best
	# A list of two-element lists
	request-headers:
	- ["User-Agent", "Mozilla/5.0"]
	fail-if-warnings: false
	dump-args: false
	ignore-args: false
	trace: false
	```
	Fields that are omitted will just have their regular
	default values. So a defaults file can be as simple as
	one line:
	``` yaml
	verbosity: INFO
	```
	Default files can be placed in the `defaults` subdirectory of
	the user data directory and used from any directory. For
	example, one could create a file specifying defaults for writing
	letters, save it as `letter.yaml` in the `defaults` subdirectory
	of the user data directory, and then invoke these defaults
	from any directory using `pandoc --defaults letter`
	or `pandoc -dletter`.

[brainchild0]

In the past when I have dealt with problems analogous to this one I have found that trying to apply a single a rule to all fields of a common basic type (e.g. lists) is generally prone to lead to confusing boundary conditions. Some list fields might be suitable handling through a list append, as you have shown, some by pre-pending, some by full replacement, and some by a case-by-case basis depending on values of other fields. It is a complex subject that is tempting to try to treat generically, though doing so is not always viable.

[jgm]

Closed by ec043e0