Simple method for providing more params to blastn

[nick-youngblut]

For pdp blastscreen, it appears that if the users blasts their primers against NCBI nt, then they will filter out the primers that hit their target taxa (assuming those taxa are in nt). I therefore made a simple addition to pdp blastscreen that allows the user to provide extra arguments to blastn such as -negative_taxids 561 in order to exclude the user's targets from the blast search. An example command would be:

pdp blastscreen -x "-negative_taxids 561" --force --db /path/to/NCBI_blastdb/nt --outdir blastn deduped_primers.json screened.json