dom/createVideo

This is a helper method to create video elements. It takes two optional arguments. The first one being an array of paths or a single path and the second one being an object of attributes for the video element.


```js
createVideo( [ "video.mp4", "video.ogg" ], { id: "video-player" } );
```
results in
```html
<video class="video-player">
    <source src="video.mp4" type="video/mp4">
    <source src="video.ogg" type="video/ogg">
</video>
```

What do you think @conradz?