make utils.extend() return the extended object

[bpierre]

It would allow to do this:

function foo (options) {
    options = utils.extend({
        a: 1 // default option
    }, options)
}

Instead of this:

function foo (optionsExt) {
    var options = {
        a: 1 // default option
    }
    utils.extend(options, optionsExt)
}

[lfac-pt]

👍 This would make it consistent with the Underscore equivalent