drop taxa functionality

[heqixin]

Thank you for developing this tool. It has been very useful for systematically retrieving genes for a group of taxa. I was wondering if you may add a function for dropping certain taxa by because of low cluster coverage (similar to the idea of drop_clstrs)? That will save quite some time for post-processing the fasta files.

[ShixiangWang]

Thank you @heqixin for your suggestion. I will take a try. I would appreciate it If you could provide a more specific and reproducible example for this.