Add ability to pass options hash to render method.

For example:

    # in the controller
    @images = DisplayCase::Exhibit.exhibit(my_obj.images)

    # in the template:
    <%= @images.render(self, locals: { my_var: "Hello" }) %>
    # then `my_var` is available for use in /images/_image.html.erb

I'm not sure how to add a test for this. Unfortunately my `mock`-foo isn't strong. If you can point me in the write direction I'll give it a shot to add a test again... I just couldn't come up with anything that seemed like it would prove this functionality out.