Improved xkcd Random Button

Fixes xkcd's unreliable "Random" button.

By default, when you click on xkcd's Random button, it will make a request to the server (c.xkcd.com/random/comic) to redirect you to a random comic. While this is usually fine, sometimes the server is slow or unresponsive. This extension eliminates the reliance on the server by instead randomly selecting a comic via a client-side script. This will ultimately result in a faster and more reliable experience. Additionally, this extension will request Chrome to prerender the randomly selected comic, which will improve its load time, making your experience even faster.