Allow parsers to modify root parameter structures, i.e. replace/remove parameters from the parsed results (i.e. during postParse)

### WIP

This is in relation to the request for the support for defining output where results are wrapped in a wrapping object that is not necessarily a class.

For example:

```javascript
{
      user: { ... } // The actual model
}
```

or even:

```javascript
{
      users: [ { ... }, { ... } ... ]  // An array of the actual underlying model.
}
```

or:

```
[ { ... }, { ... } ... ] // A collection of the underlying model.
```

Cases #1 & #2 effectively necessitates the ability for parsers to override the root parameters and substitute it with other parameters.

To demonstrate these use-cases, I added a custom parser which can be used to parse a form-type and produce parameters that almost mirrors how `FOSRestBundle` serializes form errors (but not exactly) (see: https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/2-the-view-layer.md#forms-and-views) The restructuring is done during `postParse`.

Basically I propose a non-BC change in `ApiDocExtractor#mergeParameters`: __if a `ParserInterface#parse` returns an array where an existing parameter is `NULL`, the parameter should be removed from the merged results.__ However if the parameter is not present from the parser response, it is simply preserved, which is what the current parsers are doing, anyway.