Filter counters

Adds the ability to counter how many rows were filtered out using the row filters. We need this in order to accurately calculate the number of error rows. In particular for files that don't have any data where only header and trailer records are read.

In order to satisfy backwards compatibility. I shoveled the load functions into a separate file and abstracted them.
Because it is the only one that is non obvious. I would have prefered to come up with a more meaningful name for it instead but couldn't think of one.