vigenere: New module for cyphering / decyphering

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

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

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

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