Filter contaminants

[wfondrie]

I would suggest a design change here: Instead of using FASTA file, I would merely have the user provide a list of accessions they want to exclude.

This eliminates the need to add additional dependencies and prevents problems when folks use FASTA files with unusual headers. It also simplifies this code significantly.

[ricomnl]

Nice suggestion. I changed it so that you pass in a list of contaminants to filter for and renamed the go_filters argument to go_subset to reflect the fact that the go_terms is a keep and the contaminants_filter is a throw-away argument.