Add `ez-toc-container` class to container

[tyrann0us]

Currently, the container does not include a base HTML class. So if you want to apply styles to all ToC elements regardless of their configuration, you either have to use the #ez-toc-container id (which some might consider bad practice) or use an unnecessarily inconvenient attribute selector ([class^="ez-toc-v"]). Of course, you could use the ez_toc_container_class filter to add the class yourself, but you'd need to do this on every project you use the plugin in.

By adding a generic .ez-toc-container class, none of the "workarounds" would be necessary.