Change msgHtml to allow callback as image resolver

[conrallendale]

Change the second parameter of the function msgHtml to accept a callable, beside the current string parameter. This way, I can implement my own way of getting the image data. In my case, using S3, I can request S3 for a object and return the image data, name and type. Eg:

$mail->msgHTML('<img src="my-image.jpg" />', function($url) use ($s3client) {
    $object = $s3client->getObject([ "Bucket" => "my-bucket", "Key" => $url ]);

    return [
        "name" => $url,
        "data" => (string) $object["Body"],
        "type" => $object["@metadata"]["headers"]["content-type"]
    ];
});

[Synchro]

This all looks quite reasonable - could you add a test to cover this change please?

[h00na]

Please update the var name in the comment :)

[h00na]

If the passed param is not callable we can thrown an exception telling about it. Otherwise it will be hard to debug what exactly is happening