add BLAST-based quality control filter

[gregcaporaso]

something we talked about along time ago for general quality control of reads:

given an input fasta file in post-split_libraries format:

• if good blast hit, keep
• if no good blast hit, and seq shows up in multiple samples, keep
• if no good blast hit, and seq shows up in single samples, discard

[gregcaporaso]

Imported from trac issue 42. Created by caporaso on 2011-04-25T15:34:37, last modified: 2011-04-25T15:34:47