akt password-add: add a new password to protect the wallet

Usage: akt password-add [--new-passfile=PATH] [--new-password=ARG] [--new-passenv=NAME]
                        [--counter-range MIN:MAX]

  Add a new password to protect the wallet.
  The password is added in a free key slot if there is one.
  By default the PBKDF2 iteration counter is in range 500000..1000000.
  You can change this range by using the `--counter-range` option.
  High values provide best password protection at the expense of speed.
