Function to use existing connection

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
}

Locate the FTP type and the existing connection setup, then compare them with the proposed UseConnection function in the issue. Confirm how connection errors and receive handling work in the current implementation. Done means callers can provide an existing net.Conn and receive a usable FTP instance without breaking existing connection paths.

Written by the indexing model from the issue text.