Allow editing of demos

[jzaefferer]

We want to allow users to edit our demos to explore features. That could be completely custom on just our own site, or a "Edit on jsfiddle" (or jsbin) link that creates a fresh fiddle/bin with the content of the demo, ready for the user to play with.

We don't want to deploy our content to other sites, so creating something on fiddle/bin should only happen on a user action.

[scottgonzalez]

@arschmitz says that jsbin is developing an API. Once that's released, we should look into parsing our demos and generating/updating bins, then including the "edit on jsbin" links on our sites.

[scottgonzalez]

Closing in favor of the more general issue for now: #96