make utils.extend() return the extended object

It would allow to do this:

```js
function foo (options) {
    options = utils.extend({
        a: 1 // default option
    }, options)
}
```

Instead of this:

```js
function foo (optionsExt) {
    var options = {
        a: 1 // default option
    }
    utils.extend(options, optionsExt)
}
```