Add DuckDuckGo shortcut

Added an alias ```alt + d``` to be DuckDuckGo shortcut

assets/javascripts/app/shortcuts.coffee

@@ -149,6 +149,9 @@ class app.Shortcuts
       when 40
         @trigger 'altDown'
         false
+      when 68
+        @trigger 'altD'
+        false
       when 70
         @trigger 'altF', event
       when 71

assets/javascripts/templates/pages/help_tmpl.coffee

@@ -131,6 +131,9 @@ app.templates.helpPage = ->
     <dt class="_shortcuts-dt">
       <code class="_shortcut-code">alt + s</code>
     <dd class="_shortcuts-dd">Search on Stack Overflow
+    <dt class="_shortcuts-dt">
+      <code class="_shortcut-code">alt + d</code>
+    <dd class="_shortcuts-dd">Search on DuckDuckGo
   </dl>
   <p class="_note _note-green">
     <strong>Tip:</strong> If the cursor is no longer in the search field, press <code class="_label">/</code> or

assets/javascripts/views/search/search.coffee

@@ -17,6 +17,7 @@ class app.views.Search extends app.View
     typing: 'focus'
     altG: 'google'
     altS: 'stackoverflow'
+    altD: 'duckduckgo'
 
   @routes:
     after: 'afterRoute'
@@ -113,6 +114,10 @@ class app.views.Search extends app.View
     @externalSearch "https://stackoverflow.com/search?q="
     return
 
+  duckduckgo: =>
+    @externalSearch "https://duckduckgo.com/?q="
+    return
+
   onResults: (results) =>
     @hasResults = true if results.length
     @trigger 'results', results, @flags