Support for custom states in validate_order_payment_processing

Hi,

Would you consider adding support for a filter when checking whether an order has been processed or not in Gateway.php method validate_order_payment_processing to support custom order states?

The plugin currently considers only orders in states processing and completed as processed, but we are using a custom state in our orders and that should be considered processed as well in our case. Maybe something like paytrail_processed_order_states which could default to completed and processed, but would allow adding custom states.