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;
+        }
+    }
+}

src/Discord.Net.Commands/Command.cs

@@ -25,6 +25,7 @@ namespace Discord.Commands
         public string Summary { get; }
         public string Text { get; }
         public bool HasVarArgs { get; }
+        public IReadOnlyList<string> Aliases { get; }
         public IReadOnlyList<CommandParameter> Parameters { get; }
         public IReadOnlyList<PreconditionAttribute> Preconditions { get; }
 
@@ -37,6 +38,16 @@ namespace Discord.Commands
             Name = source.Name;
             Text = groupPrefix + attribute.Text;
 
+            var aliasesBuilder = ImmutableArray.CreateBuilder<string>();
+
+            aliasesBuilder.Add(Text);
+
+            var aliasesAttr = source.GetCustomAttribute<AliasAttribute>();
+            if (aliasesAttr != null)
+                aliasesBuilder.AddRange(aliasesAttr.Aliases.Select(x => groupPrefix + x));
+
+            Aliases = aliasesBuilder.ToImmutable();
+
             var nameAttr = source.GetCustomAttribute<NameAttribute>();
             if (nameAttr != null)
                 Name = nameAttr.Text;
@@ -81,7 +92,19 @@ namespace Discord.Commands
             if (preconditionResult != null && !preconditionResult.Value.IsSuccess)
                 return ParseResult.FromError(preconditionResult.Value);
 
-            return await CommandParser.ParseArgs(this, msg, searchResult.Text.Substring(Text.Length), 0).ConfigureAwait(false);
+            string input = searchResult.Text;
+            var matchingAliases = Aliases.Where(alias => input.StartsWith(alias));
+            
+            string matchingAlias = "";
+            foreach (string alias in matchingAliases)
+            {
+                if (alias.Length > matchingAlias.Length)
+                    matchingAlias = alias;
+            }
+            
+            input = input.Substring(matchingAlias.Length);
+
+            return await CommandParser.ParseArgs(this, msg, input, 0).ConfigureAwait(false);
         }
         public Task<ExecuteResult> Execute(IMessage msg, ParseResult parseResult)
         {

src/Discord.Net.Commands/Map/CommandMap.cs

@@ -17,19 +17,21 @@ namespace Discord.Commands
 
         public void AddCommand(Command command)
         {
-            string text = command.Text;
-            int nextSpace = NextWhitespace(text);
-            string name;
-
-            if (nextSpace == -1)
-                name = command.Text;
-            else
-                name = command.Text.Substring(0, nextSpace);
-
-            lock (this)
+            foreach (string text in command.Aliases)
             {
-                var nextNode = _nodes.GetOrAdd(name, x => new CommandMapNode(x));
-                nextNode.AddCommand(nextSpace == -1 ? "" : text, nextSpace + 1, command);
+                int nextSpace = NextWhitespace(text);
+                string name;
+
+                if (nextSpace == -1)
+                    name = command.Text;
+                else
+                    name = command.Text.Substring(0, nextSpace);
+
+                lock (this)
+                {
+                    var nextNode = _nodes.GetOrAdd(name, x => new CommandMapNode(x));
+                    nextNode.AddCommand(nextSpace == -1 ? "" : text, nextSpace + 1, command);
+                }
             }
         }
         public void RemoveCommand(Command command)