Enable to add extra attributes to iframe

You can add any extra attributes to the <iframe> tag since the argument extras has been added to the IFrame class. for example:

In [1]: from IPython.display import IFrame

In [2]: print(IFrame(src="src", width=300, height=300, extras=["hello", "world"])._repr_html_())

        <iframe
            width="300"
            height="300"
            src="src"
            frameborder="0"
            allowfullscreen
            hello world
        ></iframe>

Using it, you can autoplay YouTubeVideo by adding 'allow="autoplay"', even in browsers that disable it by default, such as Google Chrome. And, you can write it more briefly by using the argument allow_autoplay.

In [1]: from IPython.display import YouTubeVideo

In [2]: print(YouTubeVideo("video-id", allow_autoplay=True)._repr_html_())

        <iframe
            width="400"
            height="300"
            src="https://www.youtube.com/embed/video-id?autoplay=1"
            frameborder="0"
            allowfullscreen
            allow="autoplay"
        ></iframe>