compressed input/output for 'merge'

[meglecz]

Modify merge to accept zipped input files (by default) and produce zipped output Make option to use unzipped I/O.

• vsearch detects automatically if its input is compressed or not. It is enough to put .gz filenames to fastqinfo.
• zip output fasta files. It seems to me, that vsearch reads gz files, but produces only unzipped output.
• pigz can be interesting for zipping with multi-threading, since files can be very large (several Gb for novaseq).