Add option for async JavaScript

[railsmechanic]

It would be great if you could define a JavaScript to be loaded asynchronously...

<script async type="text/javascript" src="/assets/app-e11efcdf30942a4f025d8093f5729ab3.js"></script>

[ixti]

I think it's not worth to have it built-in core feature. If you need custom script tag, just use asset_path:

<script async type="text/javascript" src="{% asset_path app.js %}"></script>

Anyway I'm open to discussion. If you really think it's a must have killer feature :D and can explain why do you think so, then I would consider adding it.