Preserve attribute class on img tag

[firepolo]

Allow the "class" attribute to be retained on "img" tags. This is useful for modules which could add a "class" to images and which should be included in "quill". By default Quill removes the "class" attribute from images in html.

Steps for add this

1. Goto file: packages\quill\src\formats\image.ts
2. Replace line 4 by: const ATTRIBUTES = ['class', 'alt', 'height', 'width'];

Expected behavior:
Preserve class attribute in img tag.

Actual behavior:
Remove class attribute from img tag

Improvements:
Maybe add attribute "style" for modules "quill-blot-formatter" for example.