Added support for Definition::setDefinitions() to accept callback instead of array

[biserantonov]

There are situations when I need the definitions to be generated prior to instantiation of the model, so I added the option to provide callback instead of array to setDefinitions() method. The callback is responsible to return an array of attributes definitions. For example we can use a $faker instance instead of the facade and do some calculations in isolated space before returning definitions.

$fm->define('definitionscallback:UserModelStub')->setDefinitions(function () use ($faker){
    // do some stuff here

    return [
        'name'    => $faker->word(),
        'active'  => $faker->boolean(),
        'email'   => $faker->email(),
        'age'     => $faker->numberBetween(18, 35),
        'profile' => 'factory|ProfileModelStub',
    ];
});

[GrahamCampbell]

I'm not sure I like this. Note that it's not required to use the faker facade, even without this change, you can still use our actual faker proxy object, to get the lazy behaviour.

[biserantonov]

The faker is just for example. Most often I need to execute some code which will affect several of the attributes definitions, and it's easier to wrap them all in one callback

[GrahamCampbell]

Hmmm, that is true. I'll sleep on it. :)

[biserantonov]

Here is one more suitable example:

$fm->define('Image')->setDefinitions(function () use ($faker){
    $img = $faker->image();
    list($width, $height) = getimagesize($img);
    $size = filesize($img);

    return [
        'path' => $img,
        'width' => $width,
        'height' => $height,
        'size' => $size
    ];
});