add opion to use gtf as input to generate `transTable` and `transFasta`

[yinshiyi]

Test if I can use gtf as input to generate transTable and transFasta on the fly.
This will also enable user to use their own gtf file.
Perhaps this generated the transTable

gtfToGenePred \
  Homo_sapiens.GRCh38.109.gtf \
  output.genePred