Add grep functionality for test filtering

- Use `grep` option on command line or in config file to specify a
  regular expression used to filter tests by ID
- All tests accepted by default (grep=/.*/)
- Filtered tests are skipped at runtime
- Console and runner reporters now add " (x skipped)" to the test
  results if tests were skipped.
- Console reporter now reports "x/y failed" rather than "x/y passed" so
  that the "x skipped" message will make sense, and for consistency with
  the runner reporter.