test, provide regression case for -S|--skip-illegal feature for ddiff(1)
minor, provide -S|--skip-illegal for ddiff(1)
Merge branch 'feat/ddiff-skip-illegal-lines'

* feat/ddiff-skip-illegal-lines:
  minor, provide -S|--skip-illegal for ddiff(1)
  test, provide regression case for -S|--skip-illegal feature for ddiff(1)