[dry-container] Support object instance as a decorator


Hey, release 0.7 allows to provide a decorator, but it works only if you pass a Class object. When you pass an instance it overrides the original value.
What do you think about supporting instance as a decorator and pass object being decorated to the call method of the decorator?
I already started the implementation, you can check it here: https://github.com/dry-rb/dry-container/compare/master...WojciechKo:with-decorator-instance