Medusa System Added

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

src/Nadeko.Medusa/Context/AnyContext.cs

@@ -0,0 +1,47 @@
+using Discord;
+using NadekoBot;
+
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Commands which take this class as a first parameter can be executed in both DMs and Servers 
+/// </summary>
+public abstract class AnyContext
+{
+    /// <summary>
+    /// Channel from the which the command is invoked
+    /// </summary>
+    public abstract IMessageChannel Channel { get; }
+    
+    /// <summary>
+    /// Message which triggered the command
+    /// </summary>
+    public abstract IUserMessage Message { get; }
+    
+    /// <summary>
+    /// The user who invoked the command
+    /// </summary>
+    public abstract IUser User { get; }
+
+    /// <summary>
+    /// Provides access to strings used by this medusa
+    /// </summary>
+    public abstract IMedusaStrings Strings { get; } 
+    
+    /// <summary>
+    /// Gets a formatted localized string using a key and arguments which should be formatted in
+    /// </summary>
+    /// <param name="key">The key of the string as specified in localization files</param>
+    /// <param name="args">Arguments (if any) to format in</param>
+    /// <returns>A formatted localized string</returns>
+    public abstract string GetText(string key, object[]? args = null);
+    
+    /// <summary>
+    /// Creates a context-aware <see cref="IEmbedBuilder"/> instance
+    /// (future feature for guild-based embed colors)
+    /// Any code dealing with embeds should use it for future-proofness
+    /// instead of manually creating embedbuilder instances
+    /// </summary>
+    /// <returns>A context-aware <see cref="IEmbedBuilder"/> instance </returns>
+    public abstract IEmbedBuilder Embed();
+}

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; }
+}

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; }
+}