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:

styleguide/src/templates/Comment/schema.js

Lines 81 to 99 in 879e7f6

	{
	matchMdast: matchType('linkReference'),
	props: node => ({
	identifier: node.identifier,
	url: node.url,
	referenceType: node.referenceType
	}),
	component: ({ children, identifier, url, referenceType }) => {
	if (referenceType === 'shortcut') {
	return <span>[{identifier}]</span>
	} else {
	return (
	<span>
	{children} [{identifier}]
	</span>
	)
	}
	}
	},

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.