Feature/add custom url helper

[jswift]

Due to the kinda annoying interface for add custom urls to categories, products, and brands I've add a helper that allows the following

$category = new Category();
$category->setCustomUrl('/my-slug');

$this->assertTrue($category->custom_url->is_customized);
$this->assertEquals('/my-slug', $category->custom_url->url);

[aligent-skumar]

Custom_url requires is_customized and url for brands, categories and products, assuming is_customized is supplied to the custom_url.