Add option to make subreddit traffic stats page public

Inspired by [pegasus_527](http://www.reddit.com/user/pegasus_527)'s post: [The option to make your subreddit's traffic stats publicly available](http://www.reddit.com/r/ideasfortheadmins/comments/nw7g5/the_option_to_make_your_subreddits_traffic_stats/)