Add a way to remove callbacks on the bundle

Ref. https://github.com/Sylius/Sylius/pull/1447#issuecomment-42191671

With the bundle configuration, it was impossible to remove callbacks from one configuration file to another.
This PR adds this ability thanks to a `disabled` index (relevant only for the bundle, nothing to deal with the lib part of finite) and associative array for callbacks instead of a numeric one.

See this example of what can be achieved now.
First file loaded:

    finite_finite:
        my_graph:
            ...
            callbacks:
                before:
                    before1: { on: ..., do: ... }

Second file loaded:

    finite_finite:
        my_graph:
            callbacks:
                before:
                    before1: { disabled: true }

So that the callback `before1` won't be added to the loader configuration.