Extend bank number field to allow card types to be specified

It should be possible to specify which bankcards are permitted when using the `BankcardNumberField`, eg:
```python
class BankcardForm(forms.Form):
    number = BankcardNumberField(types=[VISA_DEBIT, MASTERCARD])
```

The main use-case I've come across is excluding Amex so make sure that use-case works.