Implementation for `tap` method for the promise chain

*This is an enhancement proposal.*

Sometimes I'm looking for a clean way to debugging the responses, so I'd like to propose (and implement) a `tap` method similar to the [Bluebird](http://bluebirdjs.com/docs/api/tap.html) method.

In short:

>Essentially like .then(), except that the value passed in is the value returned.
>
>This means you can insert .tap() into a .then() chain without affecting what is passed through the chain. (See example below).

A example could be:

```js
describe('GET /user', function() {
it('responds with json', function(done) {
request(app)
.get('/user')
.auth('username', 'password')
.set('Accept', 'application/json')
.tap(response => {
// do something with response object without affecting the chain.
console.log(response);
})
.expect('Content-Type', /json/)
.expect(200, done);
});
});
```

Or better:
```js
.tap(console.log)
```

I found this can be very useful. I'd like to hear your thoughts on this.