Added a feedback plugin from uservoice.com

Right now there's an ssl error:

Uncaught SecurityError: Blocked a frame with origin "https://widget.uservoice.com" from accessing a frame with origin "http://localhost:4000".  The frame requesting access has a protocol of "https", the frame being accessed has a protocol of "http". Protocols must match.

I'm not sure if this is just because I'm on localhost or not. This adds a lot of flexability. Alternatively we could create a link on the top to submit a new issue on this github page.

Also note that you'd have to create a uservoice account (free) and change the url in index.js.