New Feature: filter barcodes/reads based on barcode quality scores

[nbokulich]

Addition Description
Filtering reads/barcodes based on average barcode Q scores is a simple method to eliminate Illumina index "cross-talk".

Current Behavior
Currently nothing in the q2-family allows filtering based on barcode quality, as far as I know.

Proposed Behavior
Filter barcodes based on average barcode Q score. Drop reads [and barcodes] paired with the bad barcode reads.

Would need both single and paired versions.

Need to report filtering summary (# reads in, # reads out, % removed — could report removals on a per-barcode basis. anything else?). Should output a filter stats visualization or print to stdout?

Comments
@ebolyen has suggested that cutadapt would be a good tool for this. However, we both agreed that this plugin would be the best home, as (a) it is a bioinformatic protocol more than a tool and relevant to the goals of this plugin and (b) it is rather similar in spirit to the q-score* methods currently available in this plugin, but examines barcodes instead of reads.