Consider adding ability to override configs

[yanovs]

We should consider adding the ability to override entire configs.

Maybe we should be able to define a new config:

class ModifiedCarConfig(dilib.Config):
    car_config = CarConfig()
    dilib.OverrideConfig(EngineConfig, NewEngineConfig)

Or we should be able to override when getting the config:

config = dilib.get_config(CarConfig, EngineConfig=NewEngineConfig)

Or when in between config and container (when we usually perturb values):

config.override_config(EngineConfig, NewEngineConfig)

Perhaps some cases can be handled by the pattern described in #13, but this would be more general because the user of a config could inject a config independently of the config writer.