refactor: Refactor command logic, and improve error handling

- Modify `delete` command to require exactly 2 arguments
- Simplify error-handling logic in retrieving secrets from vault in `password` command
- Update `upload.go` to use `os.ReadFile` instead of `ioutil.ReadFile`
- Fix error in `download` command's argument checking and change return type to `os.WriteFile`
- Rename command line argument `value` to `[value]` in `write` command