added duckduckduck go to the web-search plugin

Added duckduckgo to the list of search engines to use, including some extra !bang syntax searches like wikipedia and google maps. FYI duckduckgo is an awesome search engine: https://duckduckgo.com/about?

PS my first ever git fork! I'm not 100% sure I'm doing it right, so tell me to RTFM if I've stuffed something up.