add vsearch database search method

[gregcaporaso]

Improvement Description
This is useful for running a BLAST-like search locally. For example, I recently wanted to get a report of the best matches in my reference database for some query sequences of interest. The vsearch command looks like the following:

vsearch --db 99-otus/dna-sequences.fasta \
  --usearch_global queries.fasta \
  --alnout out.aln \
  --blast6out out.bl6 \
  --id 0.0 \
  --maxaccepts 10 \
  --qmask none

Proposed Behavior
I would see this generating a qza containing the bl6 output, which could then be viewed with qiime metadata tabulate.

[gregcaporaso]

This came up on the forum here.