Add support for spreading classes

[jquesada2016]

This is something that might be possible, but did not see it in the docs anywhere.

Would it be possible to support arbitrary class spreading, much like the attribute spread syntax?

markup::define! {
  Button<I: Iterator<Item = String>>(
    extra_classes: I,
  ) {
    button .btn ..extra_classes {
      "Click Me!"
    }
  }
}