Implement Command Aliases

This implementation could probably be more efficient, but I
opted for clarity and simplicity where possible.

src/Discord.Net.Commands/Attributes/AliasAttribute.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace Discord.Commands
+{
+    /// <summary> Provides aliases for a command. </summary>
+    [AttributeUsage(AttributeTargets.Method)]
+    public class AliasAttribute : Attribute
+    {
+        /// <summary> The aliases which have been defined for the command. </summary>
+        public string[] Aliases { get; }
+
+        /// <summary> Creates a new <see cref="AliasAttribute"/> with the given aliases. </summary>
+        public AliasAttribute(params string[] aliases)
+        {
+            Aliases = aliases;
+        }
+    }
+}