Add a way to resolve Images/Css locally

Currently it would be great if there was a way to have a CssProvider and a ImageProvider,
that can actually resolve images and fonts.

Consider a HTML with multiple Images that have relative paths like: `images/my_image.png` it would be amazing if one could resolve them locally without making a URL request, like it's done in iText via:
```java
new AbstractImageProvider() {
@Override
public String getImageRootPath() {
return environmentPath("/my_path");
}

@Override
public Image retrieve(String src) {
if (src.startsWith("/barcode/delivery/")) {
String type = src.substring(18);
return barcode((DeliveryRequest) data, writer, type);
} else {
return super.retrieve(src);
}
}
}
```

this would also give the possibility to create a image dynamically.

Same goes for CSS. (Currently the only way is to override the httpStreamImplementation, which works for Images only I guess. And a CSS should be providable without being inside the html)