Add WebContents.inspectElement to github-electron

See https://github.com/atom/electron/blob/master/docs/api/web-contents.md#webcontentsinspectelementx-y

github-electron/github-electron.d.ts

@@ -778,6 +778,10 @@ declare module Electron {
 		 * Toggle the developer tools.
 		 */
 		toggleDevTools(): void;
+		/**
+		 * Starts inspecting element at position (x, y).
+		 */
+		inspectElement(x: number, y: number): void;
 		/**
 		 * Send args.. to the web page via channel in asynchronous message, the web page
 		 * can handle it by listening to the channel event of ipc module.