Support zstd input

[mbhall88]

zstd is ever so slowly replacing gzip as the best all-round compression algorithm. It is implicitly available on most systems now and I, for one, am now using it as default when compressing my FASTQ files.

It would be a really nice feature for kraken2 to support zstd-compressed input.

Thanks, as always, for this wonderful, fast, fundamental bioinformatics tool.