/// `PosArg` represents a positional argument, i.e. one that isn't preceded
/// by a `-` or `--`. `PosArg` isn't directly used by the end application
/// writer, only internally to the `clap` library.
///
/// Example:
///
/// ```sh
/// $ myprog some_file
/// ```
///
/// where `some_file` is the first positional argument to `myprog`
///
/// **NOTE:** The index starts at `1` **NOT** `0`