custom header for css

## Proposal

Add to output config section 'header' for custom css.

## Use case

`.themekit.config.json`
```json
{
"output": {
"css": {
"header": "/* stylelint-disable */"
}
}
}
```

```bash
themekit build
```

`root.css`
```css
/* stylelint-disable */

/**
* Do not edit directly
* ......
*/

```