Change asciinema container style

From videoblock to asciinemablock to have different
styling config apart from other video players.

Fixes #38

lib/asciinema-extension.js

@@ -136,7 +136,7 @@ const toBlock = (attrs, parent, source, context, uiCatalog, defaultOptions, file
     source = doc.$apply_subs(attrs.subs, doc.$resolve_subs(subs))
   }
   const idAttr = attrs.id ? ` id="${attrs.id}"` : ''
-  const classAttr = attrs.role ? `${attrs.role} videoblock` : 'videoblock'
+  const classAttr = attrs.role ? `${attrs.role} asciinemablock` : 'asciinemablock'
 
   const block = context.$create_pass_block(parent, '', Opal.hash(attrs))