vigenere: New module for ciphering / deciphering

It just does plain (de)ciphering using filtered_string. Thus it keeps
case and spaces and every non-alpha character.

Signed-off-by: Celelibi <celelibi@gmail.com>
unvigenere: Just cipher / decipher

Cracking is not yet implemented, but still this program makes something
useful. :)

Signed-off-by: Celelibi <celelibi@gmail.com>