Medusa System Added

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

src/Nadeko.Medusa/Attributes/leftoverAttribute.cs

@@ -0,0 +1,10 @@
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks the parameter to take 
+/// </summary>
+[AttributeUsage(AttributeTargets.Parameter)]
+public class leftoverAttribute : Attribute
+{
+    
+}