This plugin parses ==custom Markdown syntax== to the HTML `<mark>` element. It adds a new node type to the [mdast](https://github.com/syntax-tree/mdast/blob/master/readme.md) produced by [remark](https://github.com/remarkjs/remark): `mark` and stringifies to HTML via [rehype](https://github.com/rehypejs/rehype) as `<mark>`.