I'd like to propose a fix to Transactions Regex pattern

Hi! I regularly use this package to parse mt940 files. However, problems appeared when I faced a negative transaction amount. It returns this error:

`booking = _parse_date( transaction.group('date')[:2] transaction.group('booking'))

AttributeError: 'NoneType' object has no attribute 'group'`

So I propose a change to your TRANSACTION_RE variable to handle negative transaction amounts. The variable would be like this:

TRANSACTION_RE = re.compile(r"""
    (?P<date>\d{6})
    (?P<booking>\d{4})?
    (?P<sign>D|C|RC|RD)
    (?P<code>\w)??  # ING skips this mandatory field
    (?P<amount>-?(\d|,){1,15})
    (?P<id>\w{4})
    (?P<reference>.{0,34})""", re.VERBOSE)

I have tested it in my local and it works fine. Please update the TRANSACTION_RE variable to handle negative transaction amounts because I depend on your package and my pipeline is breaking because of that. Thank you!