Add option to read FASTA automatically into additional-genes.fasta

[JamieHeather]

Currently it's awkward to add additional genes, both due to having to confirm to the altered IMGT header format and locating the relevant file in the squirreled away data directory.

An optional script (perhaps part of stitchrdl) could:

• be provided a path to a fasta file which it could parse
• QC the reads therein
• generate a suitable fasta header
• append the reads to the correct file