Write a script to use the rot13 (rotate 13) algorithm to encrypt/decrypt a file. rot13 adds 13 to the letters (with wrap-around). So A-M convert to N-Z and N-Z convert to A-M. "Bat" would convert to "Nmg". The first parameter should be a file to convert if present. If not present convert stdin. The second parameter should be an output file name if present. If not the output should go to stdout.