Fixing playground editing

Summary: editing numerical values didn't work. this fixes it and cleans up the API of the playground

Reviewed By: emilsjolander

Differential Revision: D6976128

fbshipit-source-id: 9b1ace9d93ba8e597bdfb4842ed6adcaf8b77fd9

website/src/templates/withPlayground.js

@@ -34,7 +34,7 @@ export default ({pathContext}) => (
               <EditValue
                 property={prop}
                 value={layout[prop]}
-                onChange={e => onChange(prop, e)}
+                onChange={onChange}
               />
             </div>
           ))}