Formats sourceMappingURL comment correctly depending on the file type

Previously, if you did source mapping for something like `sass`, the comment left in the resulting CSS would incorrectly be:

```css
//# sourceMappingURL=...
```

This is the format for Javascript source map URLS, but for CSS they [should use standard CSS comments](https://developers.google.com/chrome-developer-tools/docs/css-preprocessors):

```css
/*# sourceMappingURL=...
```

This PR formats the `sourceMappingURL` correctly for the file type. Are there any other file types we should check and handle correctly?