Added assets autoclose code to support webassets

Hello there,

I've added "assets" to the autoclose list to support [webassets](https://github.com/miracle2k/webassets) which has extensions for...

* **Flask**: [Flask-Assets](https://github.com/miracle2k/flask-assets)
* **Django**: [Django-Assets](https://github.com/miracle2k/django-assets)

It is similar to Django compressor and is the main asset manager for Flask.  Would be great to have this included if you don't mind :smile: 

Thanks
Fotis