This program encrypts a plain text message using Caesar's cipher.
The program takes in 1 command-line argument, an integer, as the cipher key. The plain text message input will be requested after the program is run.
$ ./caesar 13
plaintext: HELLO
ciphertext: URYYB
$ ./caesar 13
plaintext: be sure to drink your Ovaltine
ciphertext: or fher gb qevax lbhe Binygvar