Warn users if Simple Form is not configured in the application

If an users put Simple Form in the Gemfile and forget to configure it, it will use the default configuration included in the gem for backward compatibility.

This may lead some surprising behavior since the internal defaults are not equal to the documented default.

In the ideal world we would have both configuration matching but I prefer to keep this way to make easier to keep backward compatibility.

So, if `SimpleForm.setup` is not called in the application we will show a warning like this in console:

> [Simple Form] Simple Form was not configured in the application and will use the default values. Use `rails generate simple_form:install` to generate the Simple Form configuration.

Fixes #491