Allow iteration of configuration sections.

One issue that needs to be decided on: how does iteration work with fallback? If this configuration has a "foo" section and its parent has both a "foo" and a "bar" section, does this configuration return both "foo" and "bar" sections? (Probably.) Does the "foo" section include elements from the fallback "foo" section? (Probably not, because we are considering sections as independent entities when retrieved by themselves.