Drupal Commerce: how to add "Middle name" field to the "Address" pane? Currently for "Address" pane (field named "commerce_customer_address") available two options:

1) Name (First name, Last name) 
2) Name (single line) 

In my country middle name is very important. How to add this as separated field from custom module?

To Rayan: By the way, does it make sense to include this option in the Commerce core?
You can add the field under store -> customer profiles -> profile types -> Billing Information -> Manage Fields (Or go to admin/commerce/customer-profiles/types/billing/fields)

You can add any additional fields there. You can't add any fields directly to customer_commerce_address, but if you add any fields under the Billing Information they will show up during checkout.