Add makeSingleInstance() API to 'app' module

Electron 0.34.1 introduced a new API `makeSingleInstance()`.

https://github.com/atom/electron/blob/master/docs/api/app.md#appmakesingleinstancecallback

github-electron/github-electron.d.ts

@@ -986,6 +986,12 @@ declare module GitHubElectron {
 		setUserTasks(tasks: Task[]): void;
 		dock: BrowserWindow;
 		commandLine: CommandLine;
+		/**
+		 * This method makes your application a Single Instance Application instead of allowing
+		 * multiple instances of your app to run, this will ensure that only a single instance
+		 * of your app is running, and other instances signal this instance and exit.
+		 */
+		makeSingleInstance(callback: (args: string[], workingDirectory: string) => boolean): boolean;
 	}
 
 	interface CommandLine {