Apply consistency to attributes in the commands extension (#928)

* Apply consistency to attributes in the commands extension

This resolves #527.

Not sure if I missed any, putting this up for review.

* Allow preconditions to be used multiple times (for use with groups)

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

@@ -1,8 +1,8 @@
-using System;
+using System;
 
 namespace Discord.Commands
 {
-    [AttributeUsage(AttributeTargets.Method)]
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
     public class CommandAttribute : Attribute
     {
         public string Text { get; }