Allow a different "document_name" per value in the document

[cbeck88]

Figment has a concept of Metadata which describes the "provenance" of a value after multiple config files are composited together using their various rules.

We should allow passing some kind of closure or trait object that can describe the provenance of a value, as a String. Then we can use this string in the ConfValueSource, so that it would be used in error messages.