feat(video): support loading overlays from an attribute url

Now it is possible to insert an overlay template via the attribute `overlay-url`.

The overlay will not create a child scope at this time, and care should be taken
when using angular directives within the template to prevent issues from occurring.

Example:

```html
<video data-html5-video="media/videos/myMovie.vp8"
       data-overlay-url="media/overlays/{{mediaType}}.html">
  <p>This inline content will be erased and replaced with the loaded template
    content</p>
</video>
```