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>
```
Tried it. Turns out that `/m` isn't supported in JS. Chatted in `#javascript` for some time trying to figure out why. Tested `/m` in v0.10.28 (Mac OS X 10.9.4) and it did not work:

```
> 'foo<!-- multi-\n   line\n   test -->bar'.replace(/<!--.*?-->/gm, '');
'foo<!-- multi-\n   line\n   test -->bar'
> 'foo<!-- multi-\n   line\n   test -->bar'.replace(/<!--[\s\S]*?-->/g, '');
'foobar'
```