Module removes the id attribute in the html markup

Problem/Motivation

Sometimes we need edit HTML markup of accordion. For example add id attribute.
But the module removes the added attributes.

Steps to reproduce

1. Switched to HTML in CKEditor
2. Added to the accordion element (accordion-item) id attribute

<div class="accordion" data-accordion-id="ebb78f14dcd7c3ae771287492985f1db9">
    <div class="accordion-item" id="TEST">
        ...
    </div>
</div>

3. Switched back to Visual view in CKEditor

Proposed resolution

Implement the ability to add other attributes to the accordion markup.

Remaining tasks

User interface changes

API changes

Data model changes