Add tools/validate-spec.py script and update the Makefile
    
    You can now type "make validate" and it will run toml-validator to
    ensure all the TOML files use valid syntax.  Then it will run
    tools/validate-spec.py which is our rules as defined in TEMPLATE.toml.
    Anything not in compliance is an error.  The idea is to use this
    script to validate pull requests.  Exit code of 0 means everything is
    good.
    
    NOTE: Any changes to TEMPLATE.toml requires an update to
    validate-spec.py.  The script does not read TEMPLATE.toml to get the
    rules to enforce.  TEMPLATE.toml is meant for human reading.
    
    Signed-off-by: David Cantrell <dcantrell@redhat.com>