Add ability to set number of previously approved comments

[Vernal]

Stephen Cronin is working on a plugin to let the admin configure the number of previously approved comments necessary before a user has their new comment published. He invited us to take what we need from it to roll directly into Crowd Control. Release forthcoming.

[Vernal]

Here is the proof of concept:
https://gist.github.com/Stephen-Cronin/42f619ef9f7580924e0c#file-comment_auto_approval-php