Expose FrameRenderer so we can extend its behavior, e.g. vDOM.

[botandrose]

Hello, thank you for Turbo, I really love it! This PR is basically the exact same motivation as exposing PageRenderer in #305

With this, its trivial to replace frame rendering with a Virtual DOM like so:

import morphdom from "morphdom";
import { FrameRenderer } from "@hotwired/turbo";

FrameRenderer.prototype.loadFrameElement = function() {
  morphdom(this.currentElement, this.newElement, {
    childrenOnly: true,
    onBeforeElUpdated: function(fromEl, toEl) {
      return !fromEl.isEqualNode(toEl);
    }
  });
}

I don't believe there's an alternative, at present. If a before-frame-render event gets added, like in #431, it might be possible to do it through that.