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.