Allow fine-grained selection of sampling characters

[jessestricker]

Motivation:
Many websites have seemingly arbitrary restrictions on the set of “special characters” you can enter. This makes this app inconvenient, forcing the user to re-generate until the random password meets the websites criteria.

Also the displayed entropy with the --verbose flag is incorrect in these situations, because the user imposes additional sampling which reduces the entropy. This may mislead the user in thinking their password is more “secure” (i.e. random) than it actually is.