(1) sub-divide validation process into four phases:
    - checkResource (resource presence, encoding, decodability)
    - checkWellFormedness (XML syntax)
    - checkSchemaValidity (XSD validity assessment)
    - checkSemanticValidity (constraint validation on JAXP unmarshal results)
(2) perform foreign element and attribute pruning prior to schema
    validation and JAXP unmarshal in order to avoid false-negative
    validity reports;
(3) add new tests;
(4) improve reporting;
(5) add new command options with implementations:
    - --debug                  (debug output)
    - --debug-exceptions       (dump exception stack traces)
    - --treat-warning-as-error
    - --verbose                (increase output verbosity)
(6) add new command options with incomplete implementations:
    - --list-schemas           (list internally supported schemas)
    - --schema                 (specify schema to use)
Editorial renaming and refactoring.