Add video captions (sub-titles) extension

Add an extension.

```
class Extension(extensions.Extension):
    def handle_model(self):
        self.model.add_to_class(
            'captions',
            models.FileField(upload_to='videos/captions/', null=True, blank=True, help_text=_('Caption (sub title) file.')))

    def handle_modeladmin(self, modeladmin):
        modeladmin.add_extension_options('caption')
```

The captions would be rendered within the `<video>` tag as a `<track>` tag with a `src` attribute that points to the captions file: `<track src="/path/to/my-captions.vtt">`