1. basic_filter has a generic do-nothing __init__ method
2. more ways to specify that you aren't specifying a filter
Experimental new_filter option.  You can build a list of them in the command line through
multiple uses, e.g. --new_filter spam --new_filter eggs
Provide for specification of two datasets in one Options object, e.g. through the command line.
This uses repeated --path and --new_filter options.  We'll probably do this differently, and
better, soon.