`eslint --report-unused-disable-directives` does not work with this plugin

`eslint --report-unused-disable-directives` is not reported when a `<-- eslint-disable -->` is defined in Markdown but is unused (i.e. could be removed).

`.eslintrc.yml`:

```yml
plugins: [markdown]
rules:
no-inline-comments: 2
```

`README.md`:

````md

```js
var aa = true
```
````

`index.js`:

```js
/* eslint-disable no-inline-comments */
var aa = true
```

`package.json`:

```json
{
"name": "eslint-comments-bug",
"version": "1.0.0",
"license": "ISC",
"dependencies": {
"eslint": "^5.16.0",
"eslint-plugin-markdown": "^1.0.0"
}
}
```

In the terminal:

```bash
$ eslint --report-unused-disable-directives README.md
$ eslint --report-unused-disable-directives index.js

/home/ehmicky/eslint-comments-bug/index.js
1:1 error Unused eslint-disable directive (no problems were reported from 'no-inline-comments')

✖ 1 problem (1 error, 0 warnings)
```

`--report-unused-disable-directives` works for the JavaScript file, but not for the Markdown (where nothing is reported).