Implemented various tuning commands

// [flag], [command] for [...]
// 'C',    'X'       for Emergency stop
// 'C',    'P'       for Ping
// 'C',    'N'       for Next string
// 'C',    'S'       for Standard tuning
// 'C',    'D'       for Drop D tuning
// 'C',    'E'       for E flat tuning