Web search suggestion is loaded async

1. suggestion is async
2. if ping time of domain less than 300ms, then suggestion is still sync
3. #578 #539

Plugins/Wox.Plugin.WebSearch/SuggestionSources/ISuggestionSource.cs

@@ -2,20 +2,31 @@
 
 namespace Wox.Plugin.WebSearch.SuggestionSources
 {
-    public interface ISuggestionSource
-    {
-        List<string> GetSuggestions(string query);
-    }
-
-    public abstract class AbstractSuggestionSource : ISuggestionSource
+    public abstract class SuggestionSource
     {
+        public virtual string Domain { get; set; }
         public IHttpProxy Proxy { get; set; }
 
-        public AbstractSuggestionSource(IHttpProxy httpProxy)
+        public SuggestionSource(IHttpProxy httpProxy)
         {
             Proxy = httpProxy;
         }
 
         public abstract List<string> GetSuggestions(string query);
+
+        public static SuggestionSource GetSuggestionSource(string name, PluginInitContext context)
+        {
+            switch (name.ToLower())
+            {
+                case "google":
+                    return new Google(context.Proxy);
+
+                case "baidu":
+                    return new Baidu(context.Proxy);
+
+                default:
+                    return null;
+            }
+        }
     }
 }