append: Support chaining

I find it nicer to be able to go:

```javascript
var FormData = require('form-data'),
    form = new FormData().append('foo', 'bar').append('baz', 'quux');
```

than

```javascript
var FormData = require('form-data'),
    form = new FormData();
form.append('foo', 'bar');
form.append('baz', 'quux');
```