adding method for creating fluent accessor

This allows you to create a getter/setter that uses a fluent api, for example: (using [duo-serve](https://github.com/dominicbarnes/duo-serve) as my example here, since it's what I'm working on that called for this)

```js
delegate(Server.prototype, "settings")
  .fluent("body")
  .fluent("global")
  .fluent("html")
  .fluent("root")
  .fluent("title");
```

Which will create fluent accessors, rather than built-in getters/setters, enabling this:

```js
server()
  .body("body.html")
  .html("index.hbs")
  .title("duo-serve");
```