[web] Added preload parameter to asciinema shortcode.

web/content/docs/devguide/getting-started/build.pandoc

@@ -43,7 +43,7 @@ To speedup the compilation process append the number of cores of your cpu to the
 $ make -j 8
 ```
 
-{{< asciinema url="https://asciinema.org/a/249005" >}}
+{{< asciinema url="https://asciinema.org/a/249005" preload="1" >}}
 
 ### Option: Eclipse
 

web/layouts/shortcodes/asciinema.html

@@ -6,6 +6,7 @@
     src='{{ .Get "url" }}.js'
     async
     {{ with .Get "autoplay"}} data-autoplay="{{.}}"{{ end }}
+    {{ with .Get "preload"}} data-preload="{{.}}"{{ end }}
     {{ with .Get "loop"}} data-loop="{{.}}"{{ end }}
     {{ with .Get "speed"}} data-speed="{{.}}"{{ end }}
     {{ with .Get "cols"}} data-cols="{{.}}"{{ end }}