Add support for partial option

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

I also updated the README file to explain this option, but I don't think that is showing in this pull request. You can see it on my master branch. What can I do to include the updated documentation?