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>
```
According to [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/multiline) the `/m` only affect `^` and `$` not the *dotall* `.` In most RegExp implementations th `/s` is used for the *dotall* but that is not supported by JS (as it in the [supported flags list](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#Parameters)).

[This stack overflow answer](http://stackoverflow.com/a/1068308/227176) offers the above `[\s\S]` solution.