Bind configs to specific objects

[kalekundert]

Instead of:

config.load(obj)

The logic would go something like:

bound_config = deepcopy(config)
bound_config.bind(obj)
bound_config.load()

The advantage of this change are:

• It would make writing config classes more intuitive. The fact that a single config instance is shared between every instance of a class is surprising, and leads pretty easily to subtle bugs (e.g. shared state between instances).
• It would be easier to expose config methods in the app object, because there wouldn't be a need to inject an obj argument.
• It would be easier to replace configs at runtime. One application for this is testing.