Convert color-space FASTQ file to encoded color-space.

This is required for some tools that don't natively support color-space
files, such as BWA. For these tools, the color-space 01234 must be converted
into an encoded ACGTN.

0 -> A
1 -> C
2 -> G
3 -> T
4,5,6,. -> N

Usage: fastqutils csencode filename.fastq{.gz}