feat(article): add positional parameter for video shortcode (#331)

layouts/shortcodes/video.html

@@ -1,13 +1,14 @@
+{{- $src := .Get "src" | default (.Get 0) -}}
 <div class="video-wrapper">
     <video
     controls
+    src="{{- $src -}}"
     {{ with .Get "poster" }}poster="{{- . -}}"{{ end }}
-    {{ with .Get "src" }}src="{{- . -}}"{{ end }}
     {{ with .Get "autoplay" }}autoplay{{ end }}
     >
         <p>
             Your browser doesn't support HTML5 video. Here is a
-            <a href="{{ with .Get "src" }}{{- . -}}{{ end }}">link to the video</a> instead.
+            <a href="{{- $src -}}">link to the video</a> instead.
         </p>
     </video>
 </div>