Allows host app to configure what root scope simple_form uses for i18n.

In my organization we have multiple Rails applications sharing the same I18n assets. Each application is scoped, so we have an `apps.foo.*` scope and an `apps.bar.*` scope.

It would be nice if we could make simple_form conform to this scope and avoid mixing up keys that are tied to a particular application.

This pull request attempts to do that.