add vsearch all-pairwise-alignments pipeline

[gregcaporaso]

Improvement Description
Add vsearch all-pairwise-alignments pipeline

Current Behavior
The vsearch command I ran was:

vsearch --allpairs_global queries.fasta --acceptall --id 0.0 --blast6out out.bl6 --alnout out.aln --qmask none

Proposed Behavior
Compute all pairwise alignments for a set of query sequences

Comments

1. I would see this generating a qzv showing the pairwise alignments and maybe a heatmap-like summary of the pairwise alignment scores, and a qza containing the bl6 output that could be viewed with qiime metadata tabulate.