adding support for assets on css modules

In the core library CSS modules are not using images or any assets, but custom css modules might come with some `url()` statements, in which case the `assets` folder should be copied over the build folder, and `cssproc` should be honored.

This requirements comes from Media, where they want to build stuff like this:

```
        "stencil-css": {
            "cssfiles": [
                "base.css",
                "grid.css",
                "grid-responsive.css",
                "table.css"
            ],
            "assets": true
        }
```