Support yaml config file

[chenrui333]

I think it would be nice to support YAML config instead of JSON config

[nejch]

We do this in our package.json as a workaround so we can use YAML:

  "scripts": {
    "lint:links-yaml": "js-yaml -c .markdown-link-check.yml > .markdown-link-check.json",
    "lint:links": "yarn lint:links-yaml && markdown-link-check --quiet -c .markdown-link-check.json docs/**/*.md",
  },

But agreed it'd be nice to just have it natively ;)