Implement CFB and OFB modes of operation

[nikeee]

Like mentioned in #3 (comment), this PR tries to implement CFB and OFB modes.

You can use this for validation of the samples:

# CFB
echo "c0ldc0ffee" | python3 encrypt.py -e --iv 0123456789ABCDEF0123456789ABCDEF -m CFB examples/message.bin examples/ciphertext.10-check > /dev/null
echo "c0ldc0ffee" | python3 encrypt.py -d --iv 0123456789ABCDEF0123456789ABCDEF -m CFB examples/ciphertext.10-check examples/message.bin-check > /dev/null

sha256sum examples/message.bin examples/message.bin-check \
          examples/ciphertext.10-check examples/ciphertext.10

# OFB
echo "secret_key" | python3 encrypt.py -e --iv 0123456789ABCDEF0123456789ABCDEF -m OFB examples/message.bin examples/ciphertext.11-check > /dev/null
echo "secret_key" | python3 encrypt.py -d --iv 0123456789ABCDEF0123456789ABCDEF -m OFB examples/ciphertext.11-check examples/message.bin-check > /dev/null

sha256sum examples/message.bin examples/message.bin-check \
          examples/ciphertext.11-check examples/ciphertext.11

[mikepound]

Looking good - thanks :) It might take me a couple of days to take a proper look but i'll do it as soon as I can!