Generic filters2

I think I hit on the major points and recommendations I've gotten (but yell at me if I forgot any!). I changed it so that the filtering is done updating the original dict as much as possible and I made it clear in the documentation that that's the idea. I added back in the _done_check() method's functionality that I previously removed. I also wrote a couple tests.

So this is probably (at least from my perspective) pretty close to being mergable. Let me know what you guys think!
Do we want this to be public as well? if we see this being only useful in SFileFilter perhaps it makes sense as a method private to the class; otherwise, should there should be a filters util code file which contains these (would be cleaner, indicate to the user where new filters should go/come from and keep the size of text_processors.py down)