fast_site_remover.py with nucleic acid sequences

[aguang]

Hi, thanks for this great tool. I used fast_site_remover.py to identify my fastest evolving sites for a project I am working on, however I had nucleic acid sequences rather than amino acid sequences. I ended up modifying the script to use nucleic acid sequences and also to print out the indices of the fastest evolving sites both through optional arguments. Would these modifications be of interest to you? If so I can open a pull request with them.