fix mail validation when user sets multiple address at the same time

Hello,

I got a problem when I want to set multiple email addresses at the same time, I use $mail->AddAddress or $mail->AddBCC like this:
```
$mail->AddAddress('abc@example.com, def@example.com, ghi@example.com');
```
and I will always got an error message:
> you must provide at least one recipient email address


I traced to code and I found that was caused by the mail address validation, It won't pass the validate, and I think it does make no sense, I use this format in many mail client, in fact, if I add addresses bypass the validation, it works very well, that means the problem was the validation but nothing, even the note in **changelog.md, line 319, 'A note on sending bulk emails'** told us this can send faster and save the resource, so ...  I thought it is a bug and I did a patch here, separate the address(es) by comma, and validate each of them, if any of them failed on the validation, the validation is invalid, and I can add addresses in this format now!

So please merge this commit to fix this problem, thanks!

Regards,
Peter.