VID-15: Custom Playlist Format support

VID-5 introduces support for M3U format playlists.

However, although the intention was to introduce videojs-playlist-ui within that issue, it was quickly discovered that the plugin requires more information than we can derive from a M3U format playlist.

The relevant hooks have been commented out within embed.js

The intention of this FR is to design and implement a new playlist format so that the UI can be used.

The main criteria is that it must provide

- Name
- URL/src
- Thumbnail URL

An example of the data the plugin can consume is provided in their example implementation

    player.playlist([{
      name: 'Disney\'s Oceans 1',
      description: 'Explore the depths of our planet\'s oceans. ' +
        'Experience the stories that connect their world to ours. ' +
        'Lorem ipsum dolor sit amet, consectetur adipiscing elit, ' +
        'sed do eiusmod tempor incididunt ut labore et dolore magna ' +
        'aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco ' +
        'laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure ' +
        'dolor in reprehenderit in voluptate velit esse cillum dolore eu ' +
        'fugiat nulla pariatur. Excepteur sint occaecat cupidatat non ' +
        'proident, sunt in culpa qui officia deserunt mollit anim id est ' +
        'laborum.',
      duration: 45,
      sources: [
        { src: 'http://vjs.zencdn.net/v/oceans.mp4', type: 'video/mp4' },
        { src: 'http://vjs.zencdn.net/v/oceans.webm', type: 'video/webm' },
      ],
      // you can use <picture> syntax to display responsive images
      thumbnail: [
        {
          srcset: 'test/example/oceans.jpg',
          type: 'image/jpeg',
          media: '(min-width: 400px;)'
        },
        {
          src: 'test/example/oceans-low.jpg'
        }
      ]
    },

It might be that we want to just use that format, but having a unique format and translating across is also OK.