New "Custom URL" option for search engine. Allow stuff like 'http://yubnub.org/parser/parse?command=' or 'http://mypersonalserver.net/woot.cgi?whatever='

 * Pops up txt dialog when selected, initial default same as builtin google url
 * Active when "search" pref = 0, adds "searchurl" string pref
 * Default remains '1' (builtin google url)