[Scanner] add whitelist/blacklist roles

[larssieboy18]

Hi!

Really like the scanner cog. If possible, it would be great if it would be possible to blacklist/whitelist certain roles (just like you can already do with channels), so their messages are not processed by the cog. Might be pretty good to allow for example bots, admins and trusted users to not have to go through the moderation API. This would be especially useful for images in my opinion.