The [[Caesar Cipher]] is one of the oldest and simplest encryption techniques known. Write a filter program that encrypts or decrypts text using a Caesar Cipher. It should only shift alphabetic characters taking into account that uppercase characters should shift to other uppercase characters and lowercase characters should shift to other lowercase characters.

The program should accept two command line arguments. The first is the shift amount (0-25) and the second is the encrypt/decrypt mode. If the second argument is "e", the program should take in clear (unencrypted) text on the standard input and write cipher (encrypted) text on the standard output. If the second argument is "d", the program should take in cipher (encrypted) text on the standard input and write clear (unencrypted) text on the standard output.