Allow to modify stock allocation email subject

[chingologram]

The subject for the notification email is hardcoded in src/helpers/helper-slw-mail.php, line 41:
$subject = __('Stock allocated in', 'stock-locations-for-woocommerce') . ' ' . $term->name;

Could we add a filter as follows in line 42 ?

apply_filters('slw_stock_allocation_notification_subject', $subject, $term, $quantity, $item);

The reason for this is that we want to group notifications for the same order under the same email thread for readability purposes.

Cheers!

[fahadmahmood8]

I have added this filter line in the plugin, you will get it in the next version, thank you.