Add API that list options default value

const mariadb = require('mariadb');

console.log(mariadb.defaultOptions({ timezone: '+00:00' }));

/*

{

   host: 'localhost',

   port: 3306,

   user: 'root',

   password: undefined,

   database: undefined,

   collation: Collations.fromName('UTF8MB4_UNICODE_CI'),

   timezone: '+00:00',

   ...

}

*/