Special output format with nonce/IV.

Also added cipher key output to file.
Decryption from command line, input from file.

Can encrypt or decrypt from command line by calling `go-aes key.file
input.file output.file`, for decryption add -d flag. Removed randomly
generated input data now input comes from an input file.

Decryption extracts the nonce and encrypted data from the custom file
format.