Add support for multiple null String values

The CSVW namespace provides metadata describing a CSV file. One element of this is the ability to associate certain certain values with the null value, as recorded by the csvw:null property.

This definition corresponds (broadly) to the "null String" concept (see org.apache.commons.csv.CSVFormat#setNullString), with one noticeable difference: CSVFormat supports only a single "null String" value while CSVW, through csvw:null, supports multiple Strings.

In order to fully support CSVW, it would be helpful if CSVFormat were to be updated to allow multiple null String values.