/// `OptArg` represents a option argument for command line applications, which is one that
/// takes an additional value. Option arguments are always preceded by either a `-`
/// (single character) or `--` (single word, no spaces) then followed by a space and the
/// value. `OptArg` isn't directly used by the end application
/// writer, only internally to the `clap` library.
///
/// Example:
///
/// ```sh
/// $ myprog -a some --test other --third=file
/// ```
///
/// **NOTE:** The long version may also use the `--argument=value` version too