Extend TransformerAbstract class to easily include relations in transformers

[rdarcy1]

When building up APIs with fractal I found myself repeating the same code to include relations on my Eloquent models:

// BookTransformer.php

protected $availableIncludes = [
    'author',
    'characters'
];

public function includeAuthor(Book $book) {
     return $this->item($book->author, new AuthorTransformer);
}

public function includeCharacters(Book $book) {
    return $this->collection($book->characters, new CharacterTransformer);
}

This PR enables you to express the above code more succinctly as:

// BookTransformer.php

protected $itemIncludes = [
    'author' => AuthorTransformer::class,
];

protected $collectionIncludes = [
    'characters' => CharacterTransformer::class,
];

Tests and docs included.

[freekmurze]

Thanks you for this, but I'm not going to pull it in.

In my mind the added functionality is not worth the added complexity this would bring to the package.

[rdarcy1]

Fair enough, thanks for the great package!