Allow specification of subject prefix

- This is to permit more accurate specification of the target subject to
  be added into the RDF store in relation to the graph URI.

  For example: for indexed workspaces, only the workspace object can be
  supplied due to it being the only real object present.  If the
  supplied metadata file is nested in some directories the relative
  linkages will not work as intended.  Hence a proper resolution of the
  real subject need to be taken.
Workspace rdf indexer subject path resolution.