Add support for width and height arguments when displaying Video

[finaiized]

This PR adds width and height attributes to IPython.display.Video. The current behaviour is preserved if neither width nor height are specified. Otherwise, the specified integer will be used to set the width/height of the video in pixels, much like IPython.display.Image. This addresses the feature request asked for in #11328.

Example usage:

Video('https://archive.org/download/Sita_Sings_the_Blues/Sita_Sings_the_Blues_small.mp4', width=600, height=200)

Here's a notebook that exercises these attributes:

[Carreau]

Looks good !

As an improvement, you could try to see how to let users pass strings like 40%, 12em, but that's a good first step !