Change optMetadataFile type from Maybe to List (#5702)

Changed optMetadataFile from `Maybe FilePath` to `[FilePath]`. This allows
for multiple YAML metadata files to be added. The new default value has
been changed from `Nothing` to `[]`.

To account for this change in `Text.Pandoc.App`, `metaDataFromFile` now
operates on two `mapM` calls (for `readFileLazy` and `yamlToMeta`) and a fold.

Added a test (command/5700.md) which tests this functionality and
updated MANUAL.txt, as per the contributing guidelines.

With the current behavior, using `foldr1 (<>)`, values within files
specified first will be used over those in later files. (If the reverse
of this behavior would be preferred, it should be fixed by changing
foldr1 to foldl1.)

1 files changed, 8 insertions, 7 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index 8959d3000..56823e4b3 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -597,13 +597,14 @@ Reader options {.options}
 
 `--metadata-file=`*FILE*
 
-:   Read metadata from the supplied YAML (or JSON) file.
-    This option can be used with every input format, but string
-    scalars in the YAML file will always be parsed as Markdown.
-    Generally, the input will be handled the same as in
-    [YAML metadata blocks][Extension: `yaml_metadata_block`].
-    Metadata values specified inside the document, or by using `-M`,
-    overwrite values specified with this option.
+:   Read metadata from the supplied YAML (or JSON) file. This option can be used
+    with every input format, but string scalars in the YAML file will always be
+    parsed as Markdown. Generally, the input will be handled the same as in
+    [YAML metadata blocks][Extension: `yaml_metadata_block`]. This option can be
+    used repeatedly to include multiple metadata files; values in files specified
+    first will be preferred over those specified in later files. Metadata values
+    specified inside the document, or by using `-M`, overwrite values specified
+    with this option.
 
 `-p`, `--preserve-tabs`