Add MDX support

[darthtrevino]

Support MDX Format

MDX is a language for component-based markdown (https://mdxjs.com/). It would be fantastic if the tool could detect and lint MDX files by default

[wooorm]

Hey that’s a great idea, I never thought of that (“disclaimer:” I help maintain MDX).

Implementation on the API would look very much like the Markdown version, with one added line:

+var mdx = require('remark-mdx')

 // ...

  return core(
    value,
    unified()
      .use(markdown)
      .use(frontmatter, ['yaml', 'toml'])
+     .use(mdx)
      .use(remark2retext, makeText(config))
      .use(filter, {allow: allow})
  )