Add option to pass in container_class and container_id

[nikolalsvk]

You can now customize the element that will get replaced by the content of your request.
There's 2 ways of customizing it:

1. Pass in the container ID with container_id: 'comments-container'
2. Pass in the class name of the container element with container_class: 'comments-placeholder'

E.g.

<%= render_async some_path, container_id: 'comments-container' %>

or

<%= render_async some_path, container_class: 'comments-placeholder' %>

Closes #57