Function to use existing connection

[madman22]

I had to add a function to this library to allow using the netstack library.

func UseConnection(addr string, conn net.Conn) (*FTP, error) {
writer := bufio.NewWriter(conn)
reader := bufio.NewReader(conn)
object := &FTP{conn: conn, addr: addr, reader: reader, writer: writer, debug: false}
object.receive()
return object, nil
}