feat: add AutoReconnect option

[yi-ge]

to: #75

[yi-ge]

Test code:

eventURL := "http://localhost:8000/events.php"
client := sse.NewClient(eventURL)
client.AutoReconnect = true

// disabling ssl verification for self signed certs
client.Connection.Transport = &http.Transport{
TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
}

client.OnConnect(func(c *sse.Client) {
fmt.Println("Connected!")
})

client.OnDisconnect(func(c *sse.Client) {
fmt.Println("Disconnected!")
})

fmt.Println("Registered Server Events!")

err := client.Subscribe("messages", func(msg *sse.Event) {
if string(msg.Event) == "connected" {
	fmt.Println("Connected Event.")
} else if string(msg.Event) == "heartbeat" {
	fmt.Println("Receive heartbeat package.")
}
})

if err != nil {
fmt.Println(err.Error())
}

Console:

┌──(yige㉿kali)-[~/Project/sse-test]
└─$ go run .
Registered Server Events!
Connected!
Connected Event.
Receive heartbeat package.
Disconnected!
Connected!
Connected Event.
Receive heartbeat package.

If AutoReconnect is not set, OnDisconnect of the above instance will never be triggered.