Implement `yaml_metadata_block` extension

[jgm]

As in pandoc: https://pandoc.org/MANUAL.html#extension-yaml_metadata_block

We should avoid a yaml dependency. HsYAML might be an acceptable dependency, but an alternative could be to parse the whole block as a literal string and put it in a data-yaml attribute of a Div, so it can be extracted and processed in a second pass.