add strength command

...which returns the number of bits of entropy in the passwords we
generate. With the default length of 8, our passwords have 51 bits
of entropy.

Thanks to Darren Rhea for helping me work through the hairy set
cardinality math in this change.
remove all symbols except underscore

...and increase the default password length to 10. This change increases
the entropy of passwords of the default length by 10% (from 51 bits of
entropy to 56), and it makes them much easier to cut and paste. Since
underscore is a word character, most terminals will select an entire
password for copy/paste with a simple double-click.