“space” option to enable pretty printing (same as JSON.stringify)

Implements this feature request: https://github.com/substack/json-stable-stringify/issues/2

```js
var obj = { b: 1, a: { foo: 'bar', and: [1, 2, 3] } };
console.log(stringify(obj, { space: '\t' }));
```

outputs:

```json
{
	"a": {
		"and": [
			1,
			2,
			3
		],
		"foo": "bar"
	},
	"b": 1
}
```