Our MdYamlCodec runs inside an isolate, and send the result back. The
YamlMap data structure doesn't seem to be serializable over an isolate
and gives us an error. Therefore, we should make sure it is a normal
map.
A user requested that the YAML header be present at the bottom. I
thought this would be quite simple, but there are lots of edge cases
which will need to be solved.
Related to #223