Override icon class names

**Is your feature request related to a problem? Please describe.**
I already use a icon set, and don't want to also use Font Awesome.

**Describe the solution you'd like**
A `iconClassNames` option which provides a dictionary/map to define the icon class names.

**Additional context**
See [Bootstrap Icons](https://icons.getbootstrap.com/).
- Bold - https://icons.getbootstrap.com/icons/type-bold/
- Italic - https://icons.getbootstrap.com/icons/type-italic/
- Strike through - https://icons.getbootstrap.com/icons/type-strikethrough/
- Header 1 - https://icons.getbootstrap.com/icons/type-h1/
- Header 2 - https://icons.getbootstrap.com/icons/type-h2/
- Header 3 - https://icons.getbootstrap.com/icons/type-h3/
- Code - https://icons.getbootstrap.com/icons/code/
- Quote - https://icons.getbootstrap.com/icons/quote/
- Ordered list - https://icons.getbootstrap.com/icons/list-ol/
- Unordered list - https://icons.getbootstrap.com/icons/list-ul/
- Clean block - https://icons.getbootstrap.com/icons/eraser/
- Link - https://icons.getbootstrap.com/icons/link/
- Image - https://icons.getbootstrap.com/icons/image/
- Table - https://icons.getbootstrap.com/icons/table/
- Horizontal rule - https://icons.getbootstrap.com/icons/hr/
- Preview - https://icons.getbootstrap.com/icons/eye/
- Side-by-side - https://icons.getbootstrap.com/icons/layout-split/
- Fullscreen - https://icons.getbootstrap.com/icons/fullscreen/
- Guide - https://icons.getbootstrap.com/icons/question-circle/
- Undo - https://icons.getbootstrap.com/icons/arrow-counterclockwise/
- Redo - https://icons.getbootstrap.com/icons/arrow-clockwise/

Would be:
```javascript
iconClassNames: {
toggleBold: 'bi bi-type-bold',
toggleItalic: 'bi bi-type-italic',
toggleStrikethrough: 'bi bi-type-strikethrough',
toggleHeading1: 'bi bi-type-h1',
toggleHeading2: 'bi bi-type-h2',
toggleHeading3: 'bi bi-type-h3',
toggleCodeBlock: 'bi bi-code',
toggleBlockquote: 'bi bi-quote',
toggleOrderedList: 'bi bi-list-ol',
toggleUnorderedList: 'bi bi-list-ul',
cleanBlock: 'bi bi-eraser',
drawLink: 'bi bi-link',
drawImage: 'bi bi-image',
drawTable: 'bi bi-table',
drawHorizontalRule: 'bi bi-hr',
togglePreview: 'bi bi-eye',
toggleSideBySide: 'bi bi-layout-split',
toggleFullScreen: 'bi bi-fullscreen',
guide: 'bi bi-question-circle',
undo: 'bi bi-arrow-counterclockwise',
redo: 'bi bi-arrow-clockwise'
}
```