Added discardUnmappedColumns option that throws away extra columns that ...

...do not map to a header instead of throwing an error.

I had to introduce this options as I was working with a CSV export that would sometimes generate lines with more columns than specified in the header line. This is clearly a bug in the CSV generator, but as this was third party, I had to fix it on my end. The parser would always throw an error in this case, which will be kind of silenced by throwing away these extra columns.