Format error from the intializer

Hi,

As #894, I need to display full error messages in all my forms.
By the way, I don't like both [`#full_error`](https://github.com/plataformatec/simple_form/blob/3ea3226e2f5eb83c6723639fc10480a721347f7e/lib/simple_form/form_builder.rb#L262) and [`:error_prefix`](https://github.com/plataformatec/simple_form/blob/3ea3226e2f5eb83c6723639fc10480a721347f7e/lib/simple_form/components/errors.rb#L15) implementations

I would prefer an I18n-friendly implementation as [`full_messages_for`](http://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-full_messages_for) provided by ActiveModel

This request is only relevant for Rails 4 (and not yet covered by tests)
I guess we can find a Rails 3 way like that : fa0beb0