added fastx function, which is like sprintf for FASTA & FASTQ

[vsbuffalo]

This commit adds the function fastx, which behaves like sprintf for FASTA/FASTQ output. It simplifies FASTA/FASTQ output, as fastx($name, $seq) outputs a FASTA entry and fastx($name, $seq, $qual) outputs a FASTQ entry. I chose to make a function like sprintf because overloading a print-like function would require changing much more of the basic awk code (to deal with redirection, etc).