How to manually define place for blog post preview using Django and CKEditor?

I have a blog written in Python + Django.

Before I started use of WYSIWYG editor, to create a blog post preview I manually added custom html tag <post_cut/> and used python slice to show only a preview. It allowed to avoid issues with fixed length for preview or breaking html tags.

Now I added Django-CKEditor and it removes all html tags which "it doesn't understand".

I tried to do something with configuration (allowedContentRules, format_tags and etc.) but no success.

The questions is how to manage "post-cut" and how to do this using CKEditor.
P.S. it would be awesome also to have button for that.

Solution

Found the answer by myself.
You need to use extraAllowedContent if you want to add some extra tags.

Also found how to add custom button by creating a custom plugin.
But still looking for good solution that will utilize django-ckeditor

CKEDITOR_CONFIGS = {
    'default': {
        'extraAllowedContent': {
            'post_cut': True,
        },
        # ...
        # (other options)
    }
}