Export as Gist: Gist's description support.

This update adds a feature to the "export as Gist" command. When a Gist is exported, the bin's description from the HTML editor is read (if present) and it is sent to the [Gist API](https://developer.github.com/v3/gists/).

Descriptions can be useful in the Gist UI itself, but are especially nice if you display your Gists in a service like Mike Bostock's [bl.ocks.org](http://bl.ocks.org/).