Expand $check command to support opt-out checks for specified users

[zneix]

I would like a new type of check, it could be named optout or something similar.

Example usage:
$check optout fourtf notify => This user has opted out from remind (notify) command

Not sure how would this work, but I could possibly see something like "total optouts" of a specified user, like so:
$check optouts fourtf => This user has opted out from: remind, tuck and rl
$check optouts => You haven't opted out from anything

[Supinic]

Yes, this could definitely be an expansion of the check command. Opt-outs are public, since they can be both tested with the given user-command combo; plus, they're publically listed on each command's detail.

However, I'd like to note that blocks do not behave this way, as they're user-user specific. So they should not be included.

@zneix Would you be interested in creating a PR for this issue?