Overridable way to convert the step name into a template name

I've run into the need to dynamically choose the template rendered for a particular step. Would adding another controller method responsible for converting the step_name to a template string work? The default would be to simply pass back step_name. The user could override wizard_step_template in their controller.

```ruby
# A completely contrived example
def wizard_step_string(the_step)
  if params[:awesome_templates]
    "awesome_templates/#{the_step}"
  else
    the_step
  end
end
```

This seems easier than passing a { template: pick_template_method } options hash to every render_wizard call, but this could definitely be considered an edge case.