Added support for section (slide) "data-*" attributes

[bouzidanas]

Feature: Add <section> tag metadata ( data-* ) attributes by adding a new notebook cell metadata attribute called "data" inside the slideshow attribute:

{
    "id": "QhrtDbohUhIF",
    "slideshow": {
        "slide_type": "slide",
        "data": {
          "background_video": "video_url.mp4",
          "background_video_loop": "",
          "background_video_muted": ""
        }
    }
}

[blink1073]

Nice, thank you!