Consider adding an option to sort object keys. This could be useful when using query strings for (or as part of) a unique key, ensuring the order of property declaration doesn't affect output.

qs.stringify({ a: 'b', z: 'x' }) === 'a=b&z=x';
qs.stringify({ z: 'x', a: 'b' }) === 'z=x&a=b';
qs.stringify({ z: 'x', a: 'b' }, { sort: true }) === 'a=b&z=x';

I can work on this and provide pull request if okay.