Support MultiMarkdown metadata format

[kwin]

I would like to evaluate metadata embedded in markdown files according to the format from https://fletcher.github.io/MultiMarkdown-4/metadata.html

An extension similar to YamlFrontMatterExtension would be useful for parsing.

This would be useful e.g. in Maven Doxia which is supporting both metadata formats (but currently implements a buggy parser only supporting the most simple use cases)