Add DuckDuckGo to the bot list

Per [this page](http://www.user-agents.org/cgi-bin/free-search.cgi?template=free-search-detail.html&dbname=allagents.csv&key2=id_a_f_290308_2&action=searchdbdisplay), the DuckDuckGo user agent is ```DuckDuckBot/1.0;```