Revamped CommandsPlugin

This uses a dictionary for the commands list, if a command has a max
args set it'll only get that amount, will call the UnkownCommand event,
and now has a built in help command that can be optionally enabled.
CommandChar is now a list, but a single character can still be used.
Externally, not much should have changed, but commands can be hidden
from the help command and a description can be set. There's probably
more that I've forgotten about.

src/Discord.Net.Commands/Command.cs

@@ -9,13 +9,15 @@ namespace Discord.Commands
 		public int? MinArgs { get; internal set; }
 		public int? MaxArgs { get; internal set; }
 		public int MinPerms { get; internal set; }
-		internal readonly string[] Parts;
+        public bool IsHidden { get; internal set; }
+        public string Description { get; internal set; }
 		internal Func<CommandEventArgs, Task> Handler;
 
 		internal Command(string text)
 		{
 			Text = text;
-			Parts = text.ToLowerInvariant().Split(' ');
+            IsHidden = false; // Set false by default to avoid null error
+            Description = "No description set for this command.";
 		}
 	}
 }