Sprinkle some underscore, and fix a small bug

The main changes here are making use of `_.compose` for filter composition and `_.invoke` instead of `_.each` where applicable. Also included is a bug fix for the way `Job.addFilter()` was calling `Parser.addFilter()`.

Also note I noticed [a bug here](https://github.com/PetroFeed/roach/commit/69cbd44c4a8dd0feb110735459a64832989ff446#commitcomment-3576478), but this patch doesn't include a fix for it.