Embed blot and alt+backspace

Quill not corretly handles alt+backspace (delete last word)

**Steps for Reproduction**

1. open quilljs.com
2. go to the end of formula in editor
3. press alt+backspace
http://recordit.co/3zF3RGw0C1

**Expected behavior**:
Delete formula blot's DOM element

**Actual behavior**:
Delete only inner part of blot's DOM

**Platforms**:
mac os

**Version**:
1.3.6
