Moreover, make print_todo of PrettyPrinter have the same signature as
the Printer class. Clients must use add_filter to specify any additional
filters when printing.

The Command class holds a PrettyPrinter instance (rather than a generic
printer). This means that add_filter is called, all future invocations
of print_todo will apply these filters. This is not always desired, so
in those cases a subcommand should instantiate its own printer (this
happens for instance at the DoCommand).