Some applications treat an entire directory as a configuration, merging
the configurations from all of the files in a directory and treating them
as one.
MergeFiles supports this idea by wrapping the bodies of the several files.
It's not yet implemented here, but once implemented it will act as an
aggregator of the content of the wrapped bodies, delegating to them for
actual body content and then merging the returned body content in a
well-defined way.
