Medusa System Added

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

src/Nadeko.Medusa/Context/GuildContext.cs

@@ -0,0 +1,12 @@
+using Discord;
+
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Commands which take this type as a first parameter can only be executed in a server
+/// </summary>
+public abstract class GuildContext : AnyContext
+{
+   public abstract override ITextChannel Channel { get; }
+   public abstract IGuild Guild { get; }
+}