support different types of dice

[raspberryaether]

The "classic" Diceware algorithm calls for 7776 unique words in a list, indexed by all possible permutations of 5 six-sided dice. Using 20-sided dice, @ospalh was able to reduce the number of dice needed to index these words to three.

Unfortunately, not everyone has multiple d20s lying around. More unfortunately yet, not everyone has a d20 at all. And still more unfortunate than that is the fact that probably a great number of humans do not know what a d20 is.

Six-sided dice, however, are everywhere. Therefore, dwpwg should be extended to support dice with 6 faces.

Acceptance criteria:

• Generation of wordlists and passphrases using dice with 6 faces is supported.
• The readme is updated to mention 6-sided dice.