Add `subinfo` Slack command

[TimJentzsch]

Add a new command to Slack that allows you to get info about a given subreddit.

Usage: @Blossom subinfo <subreddit_name>

It could contain the following info:

• Total posts in the queue (and percentage of all subs)
• Current posts in the queue (and percentage of all subs)
• Total transcriptions (and percentage of all subs, percentage of in queue)
• Total volunteers transcribing that sub

[itsthejoker]

I was literally thinking about this just a few minutes ago. Are you planning on tackling it or should I take a look tonight?

[TimJentzsch]

I'm already on it :)