Release v2.2.0

Changelog

• Add a stop_parsing<> option that can be used to instruct the parser to... stop parsing. This is basically the familiar -- option, but you can also specify a different spelling if you’d like. Any unparsed options can be retrieved from the value returned by parse() by calling the unprocessed() member function.
• Fix crash when argv == nullptr. This was found through fuzz testing.
• Internal option storage now uses std::bitset instead of std::array<bool> to track what options have already been found.
• Improve error message if the clopts<> type is used with no options at all.