Add option to sort object keys

[zaim]

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.

[adam-beck]

I think a compare function would be more beneficial. Just a simple boolean would limit how the object keys are sorted.

[Marsup]

Doesn't mean it can't be both. It'll probably be faster without the function call.

[bobylito]

I need this, and I am to work on that today, if you don't mind.

[ljharb]

Fixed by #120