Media link extensions

(This page copied from https://github.com/xoofx/markdig/tree/master/src/Markdig.Tests/Specs. Used with permission.)

Adds support for media links:

Allows to embed audio/video links to popular website:

(Not a real video, so it doesn't load...)

![youtube.com](https://www.youtube.com/watch?v=mswPy5bt3TQ)

![youtube.com with t](https://www.youtube.com/watch?v=mswPy5bt3TQ&t=100)

![youtu.be](https://youtu.be/mswPy5bt3TQ)

![youtu.be with t](https://youtu.be/mswPy5bt3TQ?t=100)

![youtube.com/embed 1](https://www.youtube.com/embed/mswPy5bt3TQ?start=100&rel=0)

![youtube.com/embed 2](https://www.youtube.com/embed?listType=playlist&list=PLC77007E23FF423C6)

![vimeo](https://vimeo.com/8607834)

![static mp4](https://sample.com/video.mp4)

![yandex.ru](https://music.yandex.ru/album/411845/track/4402274)

![ok.ru](https://ok.ru/video/26870090463)
.
<p><iframe src="https://www.youtube.com/embed/mswPy5bt3TQ" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://www.youtube.com/embed/mswPy5bt3TQ?start=100" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://www.youtube.com/embed/mswPy5bt3TQ" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://www.youtube.com/embed/mswPy5bt3TQ?start=100" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://www.youtube.com/embed/mswPy5bt3TQ?start=100&amp;rel=0" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://www.youtube.com/embed?listType=playlist&amp;list=PLC77007E23FF423C6" class="youtube" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><iframe src="https://player.vimeo.com/video/8607834" class="vimeo" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>

<p><video width="500" height="281" controls=""><source type="video/mp4" src="https://sample.com/video.mp4"></source></video></p>

<p><iframe src="https://music.yandex.ru/iframe/#track/4402274/411845/" class="yandex" width="500" height="281" frameborder="0"></iframe></p>

<p><iframe src="https://ok.ru/videoembed/26870090463" class="odnoklassniki" width="500" height="281" frameborder="0" allowfullscreen=""></iframe></p>