message: Run async docs
diff: @@ -82,7 +82,9 @@ var opt = {
  "editorUrl": ...,
  "sourceHeader" : ...,
  - "sourceFooter": ...
  + "sourceFooter": ...,
  +
  + "runAsync": ...
  }
  ```
  @@ -101,6 +103,7 @@ var opt = {
  | `editorUrl` | String | The URL at which to open embedded Vega specs in a Vega editor. Defaults to `"http://vega.github.io/editor/"`. Internally, Vega-Embed uses [HTML5 postMessage](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage) to pass the specification information to the editor. |
  | `sourceHeader` | String | HTML to inject into the `` tag of the page generated by the "View Source" action link. For example, this can be used to add code for [syntax highlighting](https://highlightjs.org/). |
  | `sourceFooter` | String | HTML to inject into the end of the page generated by the "View Source" action link. The text will be added immediately before the closing `` tag. |
  +| `runAsync` | Boolean | Use [`runAsync`](https://vega.github.io/vega/docs/api/view/#view_runAsync) instead of [`run`](https://vega.github.io/vega/docs/api/view/#view_run). |
  **_This URL will be subject to standard browser security restrictions. Typically this URL will point to a file on the same host and port number as the web page itself._
  