title: Quill.js extending Blot / Parchment changes not captured in delta
body: <p>I am getting to grips with quill.js- I would like to be able to create a custom blot with content that is preset, but which I can change.. I have figured out how to create a block embed blot from the medium clone guide, but I have noticed that the created node is not being captured in the delta contents- I have set some text inside a styled div, and I would like to save those changes... I would also like to use this process for things like dynamic captions... it's important that I can save the delta. 
   node.innerText = 'test test test' sets the initial content fine, but changes are not bound to the delta.</p>
  
  <p>Is there some way to either nest blots within blocks? or can I bind the blot content with a delta somehow? any helpful example code would be greatly appreciated.  Thank you.</p>
  
  <pre><code>class EditModuleBlot extends BlockEmbed {
    static create(value) {
      let node = super.create();
      node.setAttribute('style', value.style);
      node.innerText = 'test test test';
      return node;
    }
  
    static value(node) {
      return {
        style: node.getAttribute('style')
      };
    }
  }
  EditModuleBlot.blotName = 'editmodule';
  EditModuleBlot.tagName = 'div';
  </code></pre>
  
  <p>and this is my Vue.js method for invoking it:</p>
  
  <pre><code>clickAddModule() {
    let range = this.quillInstance.getSelection(true);
    this.quillInstance.insertText(range.index, '\n', Quill.sources.USER);
    this.quillInstance.insertEmbed(range.index + 1, 'editmodule', {
      style: 'padding:10px;border: 2px dashed black;'
    }, Quill.sources.USER);
    this.quillInstance.setSelection(range.index + 2, Quill.sources.SILENT);
  }
  </code></pre>
  
  <p>The delta json does not capture the div innerText:</p>
  
  <pre><code>{
    "insert": {
      "editmodule": {
        "style": "padding:10px;border: 2px dashed black;"
      }
    }
  },
  </code></pre>
  
  <p><strong>* update *</strong>
  don't use quill for block extensions, it doesn't handle them properly--- use Slate.js or Prose Mirror or CkEditor</p>