Support password/token prompt

Closes #60

Looking for feedback, and advice on the best way to test manually entering a prompt.

I had a lot of build issues trying to use [ruby-password](http://www.caliban.org/ruby/ruby-password.shtml)

So I extracted out what was actually needed, and added it to the gem.

You can find a GitHub mirror of the source code [here](https://github.com/savonix/ruby-password/blob/master/lib/password.rb)
My guess is that it would pass `password: nil`, but don't take my word for it :wink: