Requesting pull to expose Markdown.HookCollection to node.js

```
I'd like to use the HookCollection type in some pagedown add-ons I'm writing,
but as I'm using pagedown within node.js, I currently can't access
Markdown.HookCollection as it isn't exposed in node-pagedown.js.

I've made a simple patch which just exposes Markdown.HookCollection in the same
way Markdown.Converter is exposed.

It's located at
https://bitbucket.org/ncraike/pagedown/commits/aeae84e3d59c144640de752a8736c2657
b4a1439 .

You can also pull from my fork of pagedown at
https://bitbucket.org/ncraike/pagedown . This will just include one additional
commit,
https://bitbucket.org/ncraike/pagedown/commits/4a75fb76e3af50c5bc560f2f0f0a6ff19
36acd39 , which sets HookCollection.prototype.constructor to point to the
HookCollection() function.
```

Original issue reported on code.google.com by `NCra...@gmail.com` on 7 Nov 2013 at 8:01