Allow negation in taxonomy

[mbhall88]

It would be nice to be able to provide some syntax to the -T option that effectively say logical NOT. For example, if I want genomes from family X but exclude members of genus Y. An example would follow the grep and samtools/bcftools nomenclature -T "f__Mycobacteriacea,^g__Mycobacterium"

[pirovc]

Thanks for the suggestion, it's a nice idea, I'll try to include in the next releases.