Add to support AR counter cache

* Add to support `followers_count` counter cache on followable after `follower!` and `unfollow!`.
* Add to support `likers_count` counter cache on followable after `like!` and `unlike!`.

For me, I don't think it doesn't make sense much to add support on mentionable. Open for any feedbacks.

Thank @jdewzy for his initially pull request: #22

#19