a "fluent-setter"

Running into this nice-to-have. When setting, it's `fluent`, when getting, it's a `getter`.

``` js
function Queue() {
  this.state = {
    concurrency: 1,
    timeout: false
  }
}

delegate(Queue, 'state')
   .fluent('concurrency')
   .fluent('timeout')

Queue.prototype.run = function() {
  var concurrency = this.concurrency // [Function] oh noz. 
}
```

Not sure of a name :-/
