Password confirmation

Greetings and thanks for the awesome tool, I can see getting a lot of use out of this in the future :)

This PR is borne out of me fat-fingering the encryption password the first time I set this up - which led to me trying quite a few different password variants to get the file unlocked on my home machine.

This adds a simple confirmation step to both the initial setup, and the --encrypt function on the command line.
The more idiomatic way is to write `while not confirmed_password`. But actually I'd rather name that variable `password_confirmed`, since it doesn't store a password but rather a state.