Merge pull request #244 from h-enk/video-shortcode

<div class="ratio ratio-{{ with .Get "ratio" }}{{.}}{{ end }}">

  <video{{ with .Get "attributes" }} {{ . | safeHTMLAttr }}{{ end }}>

    {{ with .Get "webm-src" -}}

    <source src="{{ . | relURL }}" type="video/webm">

    {{ end -}}

    {{ with .Get "mp4-src" -}}

    <source src="{{ . | relURL }}" type="video/mp4">

    {{ end -}}

    Sorry, your browser doesn't support embedded videos.

  </video>

</div>