Improve Fallback for Markdown References and Definitions in Comments

[tpreusse]

Currently we display node.identifier if someone uses [] in a way that triggers a linkReference, imageReference or definition.

Example in schema.js: https://github.com/orbiting/styleguide/blob/879e7f6be808bb37eff471386325f3ff746ad697/src/templates/Comment/schema.js#L81-L99

Here an example mdast: https://runkit.com/5f67c1f326b183001bb387e6/5f67c1f4be0951001a02898b

Newer version of remark seem to have a new label attribute which is better. An alternative would be to convert the children to plain text.