You can use the shortcode of the widgets like text widget with the help of this plugin. Hence, you do not need top edit the functions.php file of the theme. This plugin simply hooks shortcode_noautop and do_shortcode to the widget_text filter to make this happen. It keeps on working even if you change themes.
There are no setup options in this version. It is turned on by activating the plugin and turned off by deactivating it.