Bootstrap Buttons (for CKeditor5)

It adds a button into pool of Ckeditor 5 - Bootstrap Buttons.
Allows to configure and manage buttons to create HTML template (as a full example):

<a class="btn btn-light-primary btn-sm" href="#" target="popup">
  <em class="fa fontawesome-icon-left fa-arrow-left"></em>
  <span class="bs-icon-left glyphicon glyphicon-pencil"></span>
  <span class="text">A Button</span>
  <span class="bs-icon-right glyphicon glyphicon-pencil"></span>
  <em class="fa fontawesome-icon-left fa-arrow-right"></em>
</a>

This is analog of https://www.drupal.org/project/ckeditor_bootstrap_buttons which works only with CKEditor 4 right now (That is why I create own module to have the same tool for CKE5).

Available selectors (button classes set) may be configured on text formatting pages individually for each text format.
/admin/config/content/formats

The settings should be provided as text using next format:

Selector1
-option-label|option-value
-option-label|option-value
-option-label|option-value

Selector2
-option-label|option-value
-option-label|option-value
-option-label|option-value

For instance, default settings define three selectors: Size, Style and Color:

Size
- Small|btn-sm
- Normal|
- Large|btn-lg
Style
- Primary|btn-primary
- Secondary|btn-secondary
Color
- Light|
- Dark|dark