Update to end comment regex to support the form <!-- /ko some more text -->

In a larger project one might often end up with a lot of markup in between a start and end comment, or lots of nested comment tags.  This change relaxes constraints on parsing of the end tag, so you can include some comment text to indicate what the tag closes:

```html
<!-- ko foreach: Foo -->
... lots of lines here ...
<!-- /ko foreach: Foo -->
```