Choose when to not filter selected NSFW posts

Follow-up to #301

Rather than simply showing posts from whitelisted subreddits everywhere, the user can now specify that posts from a list of subreddits should only be whitelisted when visiting a subreddit or multi-subreddit containing a subreddit from that list.

This now supports [this use case](https://github.com/honestbleeps/Reddit-Enhancement-Suite/pull/301#issuecomment-14889817) in addition to [the other use case](https://github.com/honestbleeps/Reddit-Enhancement-Suite/pull/301#issuecomment-14889920).