Add prepend and append functions to seqtk seq

Hi,

I've come across a use case that requires the addition of a small sequence to the start and/or end of each sequence is some Illumina reads. This needs to be done quickly, so I thought I'd tack this onto `seqtk`.

This PR gives options to the subcommand `seqtk seq` that provide this functionality. I've tested it with our use case (adding restriction sites to genotyping-by-sequencing reads), and it works beautifully. It's also been valgrind'd, with no memory errors reported.

Thanks for your time,
Kevin