Enable multiple action keywords

See issue #352

Wox.Plugin/PluginMetadata.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Collections.Generic;
 
 namespace Wox.Plugin
 {
@@ -23,8 +24,11 @@ namespace Wox.Plugin
 
         public string PluginDirectory { get; set; }
 
+        [Obsolete("Use ActionKeywords instead, because Wox now support multiple action keywords. This will be remove in v1.3.0")]
         public string ActionKeyword { get; set; }
 
+        public string[] ActionKeywords { get; set; }
+
         public string IcoPath { get; set; }
 
         public override string ToString()