The flag --metashacl should be forward-ported to case_validate

[ajnelson-nist]

pyshacl provides a flag --metashacl to run the "SHACL-SHACL" test suite. This suite would be a beneficial review mechanism for users writing draft ontology updates.

The case_validate interface should add that flag to the list of flags carried forward from pyshacl's original interface.