[FEATURE] Concatenate list fields

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | parts of #129 
| Doc           | none
We could implement it like the MockBuilder in PHPUnit:
```php
Collection::create($fields, $name)
    ->setDefault('foobar')
    ->setGlue('asd')
    ->getCollection();
```
Then the object can't be changed afterwards /cc @chirimoya