filteReddit regex support, refactoring

* If filter's first item (keyword, subreddit, post domain, flair text, etc.) looks like a `/RegExp/gi`, then turn it into a regexp instead of using simple string matching
* Preprocess all the filters for efficiency on page >= 2
* Code readability cleanups

I haven't fully tested this yet, but I wanted to put it out there for critique (and maybe somebody else will do some testing too, hint hint).