Add support for autoplay and loop attributes

[grrowl]

Now I can do this:

{{% video src="future-video" autoplay="true" loop="true" %}}

having any value defined for "autoplay" or "loop" will add the properties to the video element

[grrowl]

Although just realised these are boolean attributes — so this should be an if instead of a when — ~please stand by~

edit: this is done, thanks!

[grrowl]

Added muted too, since I happened to see this on the MDN docs for video:

In some browsers (e.g. Chrome 70.0) autoplay doesn't work if no muted attribute is present.

There might be a better way handle simple boolean attributes in code (with a Map or something) but I'm a little bit new to hugo shortcodes, but happy to take suggestion. Thanks!

[martignoni]

This is not forgotten, but I didn't have the time to integrate it yet :-/

[grrowl]

Great feedback, thanks! Updated the PR