Factor the search engine installation to ease the future install of new ones.
Add DuckDuckGo search engine in the default list.
Add binding `C-c U` for DuckDuckGo search
Merge pull request #476 from ardumont/add-duckduckgo-search-engine

Add DuckDuckGo to the search engines list
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Add binding `C-c U` for DuckDuckGo search
  Add DuckDuckGo search engine in the default list.
  Factor the search engine installation to ease the future install of new ones.