Make editor examples easy to share using the editor URL

[sehilyi]

All examples in the editor are sharable via gosling.js.org/?example=doc_area. However, these URLs are not visible in the UI of the editor. Whenever a user selects an example, the browser URL could be updated (i.e., gosling.js.org → gosling.js.org/?example=doc_area) so that the user can just copy and paste the URL.