Add support for token "mutations" to enhance entropy

[cipriancraciun]

As per secpwgen manual:

ENHANCED DICEWARE METHOD
Extends the diceware method by chosing a random letter in each word
and replacing that letter with one of 32 special symbols and 4 upper-case
letters (all words in the dictionary are lower-case).

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The parameters should be:

• number of mutations;
• charset for mutations;

This should add extra token entropy, directly proportional to the initial token length (without separators), the number of mutations, and the mutation charset.

Obviously for it to have any effect, the mutation charset must be different than the original charset of the token, else no entropy is added.