Add InjectAttribute for annotating injectable properties

src/Discord.Net.Commands/Attributes/InjectAttribute.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace Discord.Commands {
+
+  [AttributeUsage(AttributeTargets.Property)]
+  public class InjectAttribute : Attribute {
+  }
+
+}

src/Discord.Net.Commands/Utilities/ReflectionUtils.cs

@@ -19,7 +19,9 @@ namespace Discord.Commands
 
             var constructor = constructors[0];
             System.Reflection.ParameterInfo[] parameters = constructor.GetParameters();
-            System.Reflection.PropertyInfo[] properties = typeInfo.DeclaredProperties.Where(p => p.CanWrite).ToArray();
+            System.Reflection.PropertyInfo[] properties = typeInfo.DeclaredProperties
+                  .Where(p => p.CanWrite && p.GetCustomAttribute<InjectAttribute>() != null)
+                  .ToArray();
 
             return (map) =>
             {