[WIP] Deconstruct -f writes off-reference fasta

[glennhickey]

Changelog Entry

To be copied to the draft changelog by merger:

• deconstruct -f option to write fasta file of off-reference sequence, as well as a tsv table describing its locations

Description

This is an old local branch I found that needs testing and merging -- Doing the PR now so I don't forget it.

[faithokamoto]

Looks like this managed to run afoul of my new scripts/check_options.py - specifically, its rule that helptext strings must wrap after 80 characters. Just pick some spot to manually wrap part of the option description onto the next line.