0.3.0

This release adds the `--retries` flag to allow users to try solving
the challenge multiple times.

BREAKING CHANGES:
  - The `-p` option no longer accepts an argument. Use `--phrase` for
    that

New features:
  - Add `--retries` to allow users to retry answering the prompt
  - Add [nix flake](https://nixos.wiki/wiki/Flakes) configuration

Changes:
  - Switch to argh parsing (exe size: 291K -> 295K)
  - The long version of `-p` is now `--mistake` option for the "I am
    probably making a mistake." prompt
  - Update rust edition to 2021
  - Update dependencies to latest
  - Use `argh` for parsin.

Bugfixes:
  - None