Enhancements - undo command

- Implemented the undo command.

src/Context.cpp

@@ -184,9 +184,7 @@ std::string Context::dispatch ()
   else if (cmd.command == "annotate")          { out = handleAnnotate        (); }
 */
   else if (cmd.command == "done")              { out = handleDone            (); }
-/*
   else if (cmd.command == "undo")              { out = handleUndo            (); }
-*/
   else if (cmd.command == "delete")            { out = handleDelete          (); }
   else if (cmd.command == "undelete")          { out = handleUndelete        (); }
   else if (cmd.command == "start")             { out = handleStart           (); }