Add alt+s shortcut for searching on Stack Overflow

Closes #174.

assets/javascripts/app/shortcuts.coffee

@@ -123,6 +123,9 @@ class app.Shortcuts
       when 82
         @trigger 'altR'
         false
+      when 83
+        @trigger 'altS'
+        false
 
   handleKeypressEvent: (event) ->
     if event.which is 63 and not event.target.value

assets/javascripts/templates/pages/help_tmpl.coffee

@@ -87,6 +87,9 @@ app.templates.helpPage = """
     <dt class="_shortcuts-dt">
       <code class="_shortcut-code">alt + g</code>
     <dd class="_shortcuts-dd">Search on Google
+    <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">escape</code>
     <dd class="_shortcuts-dd">Reset<br>(press twice in single doc mode)

assets/javascripts/views/search/search.coffee

@@ -16,6 +16,7 @@ class app.views.Search extends app.View
   @shortcuts:
     typing: 'autoFocus'
     altG: 'google'
+    altS: 'stackoverflow'
 
   @routes:
     root: 'onRoot'
@@ -86,12 +87,21 @@ class app.views.Search extends app.View
     @removeClass @constructor.activeClass
     @trigger 'clear'
 
-  google: =>
-    if @value
-      $.popup "https://www.google.com/search?q=#{encodeURIComponent @value}"
+  externalSearch: (url) ->
+    if value = @value
+      value = "#{@scope.name()} #{value}" if @scope.name()
+      $.popup "#{url}#{encodeURIComponent value}"
       @reset()
     return
 
+  google: =>
+    @externalSearch "https://www.google.com/search?q="
+    return
+
+  stackoverflow: =>
+    @externalSearch "https://stackoverflow.com/search?q="
+    return
+
   onResults: (results) =>
     @hasResults = true if results.length
     @trigger 'results', results, @flags

assets/javascripts/views/search/search_scope.coffee

@@ -26,6 +26,9 @@ class app.views.SearchScope extends app.View
   getScope: ->
     @doc or app
 
+  name: ->
+    @doc?.name
+
   search: (value) ->
     unless @doc
       @searcher.find app.docs.all(), 'text', value