Add `--zotu-identity` parameter for zOTU table generation step

[mhoban]

This resolves #85, for cases where users really need to customize this.

[mhoban]

This update adds a --zotu-identity parameter that controls the --id (vsearch) or -id (usearch) parameter in the zOTU table generation step.

It also adds a zotu_map.tsv file to the dereplication output. This file shows which raw reads (.number) are mapped to which zOTU, and theoretically allows some forensic investigation.