Add support for partial option when no block is used with fields_for

Allow for a :partial option when not using a block in form_for. Useful in case you want to use something besides the "#{association_name.to_s.singularize}fields" partial. For example, if you have a polymorphic association in which the nested fields would be the same throughout the application. It's cleaner than the suggestion here: https://github.com/ryanb/nested_form/issues/167