New CLI option to pass YAML metadata

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A miss a (repeatable) option to pass a YAML file with metadata, e.g. --yaml-metadata=FILE. This would be handy to separate document body and additional fields such as author, date, abstract, keywords etc. Right now one can only set single metadata values with --metadata KEY[=VAL],

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I found out that YAML files can be used as simple input files, so there is no need for an option:

catmandu metadata.yml document.md

This could be mentioned in the documentation.