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.
Merge branch 'master' of github.com:WaYdotNET/prelude

* 'master' of github.com:WaYdotNET/prelude:
  update js