GithubRun.user.js

// ==UserScript==
// @name        GithubRun
// @namespace   http://www.github.com
// @description Adds a Live Run button to Github.com source files
// @include     https://github.com/*/*
// @version     1
// @require     https://ajax.googleapis.com/ajax/libs/jquery/2.2.3/jquery.min.js
// @grant       none
// ==/UserScript==

(function () {
	run();
 
	function run() {
		setupAjaxInterception();
		addRunMenu();
	}
	
	function setupAjaxInterception() {
		$(document).ajaxSuccess(function(event, xhr, settings) {
			if (settings.url.indexOf("blob") !== -1) {
				console.log( "Triggered ajaxSuccess handler. The ajax url was: " + settings.url);
				addRunMenu();
			}
		});
	}
	
	function addRunMenu() {
		var $rawButton = $("#raw-url:not(.rawgithub-added)");

		if ($rawButton.length === 0) {
			return;
		}
		
		var updatedHref = "http://rawgit.com" + $rawButton.attr("href").replace("/raw", "");
		
		$rawButton.clone()
			.attr("href", updatedHref)
			.attr("id", "rawgithub-raw-url")
			.text("Live Run")
			.attr("title", "Clicking this button will open this page via rawgithub.com")
			.addClass("tooltipped")
			.prependTo($rawButton.parent())
		;

		$rawButton.addClass("rawgithub-added");
	}
})();