seqkit v0.3.0

_64-bit versions are highly recommended._

Changelog

- use fork of github.com/brentp/xopen, using `zcat` for speedup of .gz file
  reading on \*nix systems.
- improve speed of parsing sequence ID when creating FASTA index
- reduce memory usage of `seqkit subseq --gtf`
- fix bug of `seqkit subseq` when using flag `--id-ncbi`
- fix bug of `seqkit split`, outdir error
- fix bug of `seqkit seq -p`, last base is wrongly failed to convert when
  sequence length is odd.
- add "sum_len" result for output of `seqkit stat`