ENH: action to get preformatted blast databases

[nbokulich]

NCBI hosts a database of preformatted blast databases, and a tool to download these programmatically (at the moment this seems to be a perl script, update_blastdb.pl, which ships with blast+).

It would be neat to add an action to RESCRIPt for downloading these databases (basically, this would allow quick validation of the source and timestamp in provenance).

One blocker at the moment is the lack of a BLASTDB type/format in Q2 — related to qiime2/q2-feature-classifier#158

[colinbrislawn]

Direct link: https://ftp.ncbi.nlm.nih.gov/blast/temp/update_blastdb.pl

Not sure if we want to wrap this script or just port part of it