Multi-line comment support

The comment Regexp was not capturing comments that spanned more than one line (includes a `\n` in them). Using the `[\s\S]` pattern it will span over `\n` as if they were part of the comment pattern.

This PR includes a test case.

### Example

```xml
<root>
  <!-- Multi-
       line
       comment -->
  <a>foobar</a>
</root>
```
adding `/m` should work as well but meh! LGTM