Configurable form_builder in config

[fschwahn]

I have some custom behavior which applies to all forms in my app. I can either follow https://github.com/plataformatec/simple_form#custom-form-builder (and replace all simple_form_for-calls) or monkeypatch SimpleForm::FormBuilder, but I was thinking if it might make sense to make a custom form_builder configurable. This would allow users to define something like a ApplicationFormBuilder (analogous to ApplicationRecord etc.) which has app-specific behavior. Configuration would simply be:

SimpleForm.setup do |config|
  config.form_builder = "ApplicationFormBuilder"
end

I'd be willing to put a PR together, but I first wanted to get a sense if this would be considered a useful addition?