Add custom ping message option to Ping function

```
func (c *Conn) Ping(ctx context.Context) error {
p := atomic.AddInt32(&c.pingCounter, 1)

err := c.ping(ctx, strconv.Itoa(int(p)))
if err != nil {
return fmt.Errorf("failed to ping: %w", err)
}
return nil
}

func (c *Conn) ping(ctx context.Context, p string) error {
```

Is it possible to add a parameter to use custom ping/pong message when sending it? Sometimes, one side want to listen to specific ping/pong message. If we can add that, it would be very helpful.
I really like this project. It is simple enough that I can just focus my project logic.