Adds VIPParametersCheck class to identify bad parameter values in function calls

Addresses issue #46 
I mainly did this since VIPRestrictedPatternsCheck.php put the check directly in the slug parameter, then tested for the presence of a specific slug in the unit tests for that class. If I give it a more generic slug, I would just need to update the unit tests to look for that generic slug. As far as I can see, the slug does not even make it into the HTML output for the scanner results. Mainly seems to be there for internal use.