tracker-extract: Add dummy extractor

For use with data types that don't have any additional metadata
inside the file, but need tagging with specific RDF types.

Note that the use of a dummy extractor is only temporary, and rule files
should instead allow not having a ModulePath entry.
See https://bugzilla.gnome.org/show_bug.cgi?id=735610

https://bugzilla.gnome.org/show_bug.cgi?id=735460