Add ABOR command

DearFTP/Connection/Commands/AbortCommand.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace DearFTP.Connection.Commands
+{
+    class AbortCommand : ICommand
+    {
+        public string[] Aliases { get; } = new string[]
+        {
+            "ABOR"
+        };
+
+        public void Execute(Session session, FtpStream stream, string alias, string argument)
+        {
+            session.DataConnection.Close();
+
+            stream.Send(ResponseCode.CloseDataConnection, "Data connection closed.");
+        }
+    }
+}

DearFTP/Connection/Commands/CommandsDispatcher.cs

@@ -7,6 +7,7 @@ namespace DearFTP.Connection.Commands
     {
         public ICommand[] Commands { get; } = new ICommand[]
         {
+            new AbortCommand(),
             new AuthCommand(),
             new ClntCommand(),
             new CwdCommand(),