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.)

0 files changed, 0 insertions, 0 deletions