incompatible with markdown files and eslint-plugin-mdx out of box

[JounQin]

Similar to #411, this is because of eslint-plugin-markdown@1

.md are parsable as estree by eslint-plugin-mdx and eslint-plugin-markdown@2 has changed to use virtual filename instead, so the md to babel mapping becomes incorrect then.

I don't know if you think this as BREAKING CHANGE for eslint-plugin-markdown@1 users, but they can simply set parser: 'markdown' if we remove the incorrect mapping.