Adding a flag for `PuzzlePart` in `read` subcommand

Hi. 👋
Awesome cli! ⭐

Would you be interested in a PR that adds a cli flag that specifies the part/level for a puzzle for the `read` subcommand?

There is a request for this in https://github.com/fspoettel/advent-of-code-rust/issues/50, so I was wondering if it would make sense for this feature to be added in here.