Medusa System Added

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

src/Nadeko.Medusa/Context/DmContext.cs

@@ -0,0 +1,11 @@
+using Discord;
+
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Commands which take this type as the first parameter can only be executed in DMs
+/// </summary>
+public abstract class DmContext : AnyContext
+{
+    public abstract override IDMChannel Channel { get; }
+}