Add support for reactions

I'd love to reply to a message / command, not by text, but by reaction.

I'm trying to look into this myself, but I'm not experienced enough yet

eg:
```
$botman->hears('wave', function (BotMan $bot)
{
$bot->replyWithReaction("thumbsup");
});
```

https://api.slack.com/methods/reactions.add

I suppose it would need to be added here
https://github.com/botman/driver-slack/blob/a832671a0bc2a42b083ebaea60ea7a4efee96045/src/SlackDriver.php#L258