[Feature Request] Add Special Character Set

[bennokress]

Some sites require special characters, but only from a specified set. It would be nice to be able to use the following syntax to generate a password of length 20 with at least one of the specified characters (but no others) randomly used pw 20 &/.-. It would make sense to change option 2 for this feature to go from 20 character password (no special characters) to 20 character password (using special characters from '&/.-').