Create render_react_component controller helper

`render_react_component` allows you to render a React component in place of a typical rails template from `app/views`.

Instead of creating a view that only renders a component, _e.g._
```erb
<!-- app/views/todos/index.html.erb -->
 
<%= react_component "TodoList", {:todos => @todos}, :prerender => true %>
```
You can create call the helper from the controller action instead, _e.g._
```ruby
# app/controllers/todos_controller.rb

class TodosController < ApplicationController
  def index
    @todos = %w{todo1 todo2 todo3}
    render_react_component "TodoList", {:todos => @todos}, :prerender => true
  end
end
```
Should this go in an active support hook? 

See for example [how react-rails includes the view helper](https://github.com/reactjs/react-rails/blob/master/lib/react/rails/railtie.rb#L23), also here's an [ActionController example](http://pothibo.com/2013/6/customize-rails-view-controller-with-railtie)