Support for legacy account numbers and bank IDs

In our database we have a lot of legacy bank account information from the pre-IBAN era. Additionally, a lot of customers are not yet used to providing IBAN instead of a legacy account number. We added extra functionality to deal with this:

* `IBAN.bank_id` and `IBAN.account_number` for extracting the country-specific legacy bank identifier and account number from IBAN. For each country different rules can be specified in `rules.yml`.
* `IBAN.generate_check_digits` for generating IBAN from a legacy account numer.