Ing csv

This is to handle another special case of csv. ING provides me with csv file that contain a "sign" column, i.e. that column tells you it it was added or removed from the account. I think that this is likely to be quite common for csv, but without other examples I am not sure how general my solution is. One big problem is how to detect whether the term means to add or to substract. For the Dutch csv "Bij" is to add and "Af" is to substract. In english credit and debit are probably mostly used. 

It might be better to require a command line option for this, but I couldn't think of a good term. Furthermore we would need multiple options (one for which column, one for which word means adding to the account (in the ING case "Bij")).

Please let me know what you think. These current changes work for me and all tests are still passed, so it doesn't break anything.