Add the ability for users to specify the Google domain to use for the search.
Done per a user request that landed in plugins.guifications.org's spam bin.
/*
* irssi - Implements several irssi features for Purple
* Copyright (C) 2005-2008 Gary Kramlich <grim@reaperworld.com>
* Copyright (C) 2006-2008 John Bailey <rekkanoryo@rekkanoryo.org>