added a parameter to transform URLs without adding assets to webpack pip...

...eline

Hi, I understand that it may be not the webpack way but this is what I am using in my current project.

The use case is when you have resources referred from css file: `url(my-image)` and you just want to deploy them to a CDN (task outside of webpack concern) and change the resulting CSS to `url(http://my-cdn/hash/path/my-image)`.

I wonder if this may be useful to others.