Ability to supply configuration arguments to backends

[vaaale]

I have written a new HTML backend to provide better support for parsing HTML files. My implementation also supports extracting images. The add_picture method on the Document take in an ImageRef argument that can either be just an URI or the actual PIL image. I would like to be able to configure which strategy is used as part of configuring the pipeline, but I can't see any way to supply such configuration parameters to the backend. I could of course just have two backend implementations inheriting from the same base class and configure it that way, but maybe there is a better way?