message: Document why we use `AppSettings`.
diff: @@ -17,7 +17,10 @@ use test::{self, webdriver};
  #[derive(Debug, Default, StructOpt)]
  #[structopt(
  + // Allows unknown `--option`s to be parsed as positional arguments, so we can forward it to `cargo`.
  setting = AppSettings::AllowLeadingHyphen,
  +
  + // Allows `--` to be parsed as an argument, so we can forward it to `cargo`.
  setting = AppSettings::TrailingVarArg,
  )]
  /// Everything required to configure the `wasm-pack test` command.
  