Update flash message default key matching

Most used flash keys are `notice` and `alert`. They are used respectively for notice messages, for example after a successful update, and for error messages, for example after a validation error.

These keys are also the only direct flash parameters used by the `redirect_to` method, see https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/flash.rb

That's why foundation's `alert` class is now associated with the standard `alert` flash key instead of the non-standard `failure` key.

Anyway now you can use a hash parameter to override default behavior with custom key matching, see specs for more informations.