Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/NadekoBot/Common/NoPublicBotPrecondition.cs

@@ -18,4 +18,13 @@ public sealed class NoPublicBotAttribute : PreconditionAttribute
         return Task.FromResult(PreconditionResult.FromSuccess());
 #endif
     }
+}
+
+/// <summary>
+/// Classed marked with this attribute will not be added to the service provider 
+/// </summary>
+[AttributeUsage(AttributeTargets.Class)]
+public class DontAddToIocContainerAttribute : Attribute
+{
+    
 }