Added discounts support

Configuration example
```php
        'cart' => [
            'class' => 'yz\shoppingcart\ShoppingCart',
            'discounts' => [
                'frontend\modules\store\discounts\CardDiscount'
            ],
        ],
```

Discount example

```php
namespace frontend\modules\store\discounts;

use Yii;
use yz\shoppingcart\Discount;

class CardDiscount extends Discount
{
    public function apply()
    {
        if (Yii::$app->session->has('card')) {
            foreach($this->shoppingCart->getPositions() as $position) {
                $position->setDiscountPrice($position->getCost(false) * (Yii::$app->session->get('card') / 100));
            }
        }
    }
} 
```