Add proper support for config dispatching

[yanovs]

Here's an interesting pattern that just works right now:

import dilib


class CarConfig(dilib.Config):
    engine_cfg_a = EngineConfigA()
    engine_cfg_b = EngineConfigB()
    engine_cfg = dilib.Forward(engine_cfg_b)

    engine = dilib.Forward(engine_cfg.engine)

We should:

• Add unit tests for this
• Document the pattern in the README
• It actually almost just works: we need to fix perturbing the engine cfg