More options for password generation

[cohen990]

some websites with particularly poor security will require you to only use alphabetic characters. Could be valuable to have an command line option like '-D' which will create a password without digits

Once genpass supports special characters this will become particularly relevant

[cyplo]

Currently genpass provides this kinda by accident - if you enable all options but digits it effectively removes digits from the character set, i.e. genpass -lu. This is not clear though that this is the behavior and I agree that we should provide something more explicit.