Add ability to use Markdown in attachments

[Repox]

Addresses issue #30 (I created myself).

Usage:

$message = $client->getMessageBuilder()
            ->setText('')
            ->setChannel($channel)
            ->addAttachment(new Attachment('Title', 'This *is* _formatted_', 'danger, 'fallback', [], ['text', 'pretext', 'fields']))
            ->create();