[FEATURE] Add CollectionViewHelper

It's the first time I try to commit something on Github, so help me out if I'm doing something really wrong.

I didn't know to what category to add this VH to, so feel free to relocate it.
> how do you suggest to query by an array of uid's?

Usually you would do:

```php
$query = $repository->createQuery();
$query->matching($query->in('uid', $uidListArray));
$results = $query->execute();
```

I understand the wish for the `findAll` support but we must view these ViewHelpers in two separate contexts: one being the controller-less templates you find in fluidcontent and fluidpages, the other being real controller contexts belonging to true Extbase plugins. In the first context a `findAll` is not desirable for two reasons:

1. When you do operate on Collections you **should** do so by a relation selector field and not render anything if not selected. More advanced use **should** be created as a true Extbase plugin, with a proper controller. VHS's ViewHelpers are not a replacement for true plugins - and as such, we do not want to open doors to many more requirements such as filtering by category and other things **unless** this is done as TCA `foreignTableWhere` settings on your relation fields. In other words: the ViewHelper will be *a tool to render already selected, specific Collections* - anything else should be a plugin.
2. When using a plugin there is a proper controller and it is in this controller that any `findAll` should happen if for example no Collection is selected.

Hopefully that explains the rejection of `findAll` as part of this ViewHelper ;)